diff --git src/wp-admin/includes/ajax-actions.php src/wp-admin/includes/ajax-actions.php
index 9d8752caea..d44fd05c4c 100644
--- src/wp-admin/includes/ajax-actions.php
+++ src/wp-admin/includes/ajax-actions.php
@@ -2532,7 +2532,7 @@ function wp_ajax_upload_attachment() {
 	check_ajax_referer( 'media-form' );
 	/*
 	 * This function does not use wp_send_json_success() / wp_send_json_error()
-	 * as the html4 Plupload handler requires a text/html content-type for older IE.
+	 * as the html4 Plupload handler requires a text/html Content-Type for older IE.
 	 * See https://core.trac.wordpress.org/ticket/31037
 	 */
 
diff --git src/wp-admin/includes/file.php src/wp-admin/includes/file.php
index e8fb7a6951..b82d128b0d 100644
--- src/wp-admin/includes/file.php
+++ src/wp-admin/includes/file.php
@@ -1184,7 +1184,7 @@ function download_url( $url, $timeout = 300, $signature_verification = false ) {
 		return new WP_Error( 'http_404', trim( wp_remote_retrieve_response_message( $response ) ), $data );
 	}
 
-	$content_disposition = wp_remote_retrieve_header( $response, 'content-disposition' );
+	$content_disposition = wp_remote_retrieve_header( $response, 'Content-Disposition' );
 
 	if ( $content_disposition ) {
 		$content_disposition = strtolower( $content_disposition );
@@ -1211,7 +1211,7 @@ function download_url( $url, $timeout = 300, $signature_verification = false ) {
 		}
 	}
 
-	$content_md5 = wp_remote_retrieve_header( $response, 'content-md5' );
+	$content_md5 = wp_remote_retrieve_header( $response, 'Content-MD5' );
 
 	if ( $content_md5 ) {
 		$md5_check = verify_file_md5( $tmpfname, $content_md5 );
@@ -1238,7 +1238,7 @@ function download_url( $url, $timeout = 300, $signature_verification = false ) {
 
 	// Perform signature valiation if supported.
 	if ( $signature_verification ) {
-		$signature = wp_remote_retrieve_header( $response, 'x-content-signature' );
+		$signature = wp_remote_retrieve_header( $response, 'X-Content-Signature' );
 
 		if ( ! $signature ) {
 			// Retrieve signatures from a file if the header wasn't included.
diff --git src/wp-admin/includes/upgrade.php src/wp-admin/includes/upgrade.php
index 5030483769..923455d82a 100644
--- src/wp-admin/includes/upgrade.php
+++ src/wp-admin/includes/upgrade.php
@@ -529,7 +529,7 @@ function wp_install_maybe_enable_pretty_permalinks() {
 		 * can block head requests.
 		 */
 		$response          = wp_remote_get( $test_url, array( 'timeout' => 5 ) );
-		$x_pingback_header = wp_remote_retrieve_header( $response, 'x-pingback' );
+		$x_pingback_header = wp_remote_retrieve_header( $response, 'X-Pingback' );
 		$pretty_permalinks = $x_pingback_header && get_bloginfo( 'pingback_url' ) === $x_pingback_header;
 
 		if ( $pretty_permalinks ) {
diff --git src/wp-includes/Requests/Transport/fsockopen.php src/wp-includes/Requests/Transport/fsockopen.php
index 56f94c99f2..fc9f6d2a52 100644
--- src/wp-includes/Requests/Transport/fsockopen.php
+++ src/wp-includes/Requests/Transport/fsockopen.php
@@ -163,7 +163,7 @@ class Requests_Transport_fsockopen implements Requests_Transport {
 				$request_body = $data;
 			}
 
-			// Always include Content-length on POST requests to prevent
+			// Always include Content-Length on POST requests to prevent
 			// 411 errors from some servers when the body is empty.
 			if (!empty($data) || $options['type'] === Requests::POST) {
 				if (!isset($case_insensitive_headers['Content-Length'])) {
diff --git src/wp-includes/SimplePie/Misc.php src/wp-includes/SimplePie/Misc.php
index ce3cf0f546..e8a4fa5e1b 100644
--- src/wp-includes/SimplePie/Misc.php
+++ src/wp-includes/SimplePie/Misc.php
@@ -2146,7 +2146,7 @@ class SimplePie_Misc
 		{
 			ob_start('ob_gzhandler');
 		}
-		header('Content-type: text/javascript; charset: UTF-8');
+		header('Content-Type: text/javascript; charset: UTF-8');
 		header('Cache-Control: must-revalidate');
 		header('Expires: ' . gmdate('D, d M Y H:i:s', time() + 604800) . ' GMT'); // 7 days
 		?>
diff --git src/wp-includes/class-json.php src/wp-includes/class-json.php
index 9eca6f4b57..8e2cb7ca90 100644
--- src/wp-includes/class-json.php
+++ src/wp-includes/class-json.php
@@ -291,7 +291,7 @@ class Services_JSON
     {
         _deprecated_function( __METHOD__, '5.3.0', 'The PHP native JSON extension' );
 
-        header('Content-type: application/json');
+        header('Content-Type: application/json');
         return $this->encodeUnsafe($var);
     }
     /**
diff --git src/wp-includes/class-simplepie.php src/wp-includes/class-simplepie.php
index a392e54cd9..3d639fc709 100644
--- src/wp-includes/class-simplepie.php
+++ src/wp-includes/class-simplepie.php
@@ -1864,7 +1864,7 @@ class SimplePie
 	}
 
 	/**
-	 * Send the content-type header with correct encoding
+	 * Send the Content-Type header with correct encoding
 	 *
 	 * This method ensures that the SimplePie-enabled page is being served with
 	 * the correct {@link http://www.iana.org/assignments/media-types/ mime-type}
@@ -1886,7 +1886,7 @@ class SimplePie
 	{
 		if (!headers_sent())
 		{
-			$header = "Content-type: $mime;";
+			$header = "Content-Type: $mime;";
 			if ($this->get_encoding())
 			{
 				$header .= ' charset=' . $this->get_encoding();
diff --git src/wp-includes/class-snoopy.php src/wp-includes/class-snoopy.php
index fb6857c525..e82c673989 100644
--- src/wp-includes/class-snoopy.php
+++ src/wp-includes/class-snoopy.php
@@ -53,7 +53,7 @@ class Snoopy
 	var $cookies		=	array();			// array of cookies to pass
 												// $cookies["username"]="joe";
 	var	$rawheaders		=	array();			// array of raw headers to send
-												// $rawheaders["Content-type"]="text/html";
+												// $rawheaders["Content-Type"]="text/html";
 
 	var $maxredirs		=	5;					// http redirection depth maximum. 0 = disallow
 	var $lastredirectaddr	=	"";				// contains address of last redirected address
@@ -825,13 +825,13 @@ class Snoopy
 				$headers .= $headerKey.": ".$headerVal."\r\n";
 		}
 		if(!empty($content_type)) {
-			$headers .= "Content-type: $content_type";
+			$headers .= "Content-Type: $content_type";
 			if ($content_type == "multipart/form-data")
 				$headers .= "; boundary=".$this->_mime_boundary;
 			$headers .= "\r\n";
 		}
 		if(!empty($body))
-			$headers .= "Content-length: ".strlen($body)."\r\n";
+			$headers .= "Content-Length: ".strlen($body)."\r\n";
 		if(!empty($this->user) || !empty($this->pass))
 			$headers .= "Authorization: Basic ".base64_encode($this->user.":".$this->pass)."\r\n";
 
@@ -990,12 +990,12 @@ class Snoopy
 		}
 		if(!empty($content_type)) {
 			if ($content_type == "multipart/form-data")
-				$headers[] = "Content-type: $content_type; boundary=".$this->_mime_boundary;
+				$headers[] = "Content-Type: $content_type; boundary=".$this->_mime_boundary;
 			else
-				$headers[] = "Content-type: $content_type";
+				$headers[] = "Content-Type: $content_type";
 		}
 		if(!empty($body))
-			$headers[] = "Content-length: ".strlen($body);
+			$headers[] = "Content-Length: ".strlen($body);
 		if(!empty($this->user) || !empty($this->pass))
 			$headers[] = "Authorization: BASIC ".base64_encode($this->user.":".$this->pass);
 
diff --git src/wp-includes/comment.php src/wp-includes/comment.php
index 4777b91dff..ddf3f208fa 100644
--- src/wp-includes/comment.php
+++ src/wp-includes/comment.php
@@ -2802,12 +2802,12 @@ function discover_pingback_server_uri( $url, $deprecated = '' ) {
 		return false;
 	}
 
-	if ( wp_remote_retrieve_header( $response, 'x-pingback' ) ) {
-		return wp_remote_retrieve_header( $response, 'x-pingback' );
+	if ( wp_remote_retrieve_header( $response, 'X-Pingback' ) ) {
+		return wp_remote_retrieve_header( $response, 'X-Pingback' );
 	}
 
 	// Not an (x)html, sgml, or xml page, no use going further.
-	if ( preg_match( '#(image|audio|video|model)/#is', wp_remote_retrieve_header( $response, 'content-type' ) ) ) {
+	if ( preg_match( '#(image|audio|video|model)/#is', wp_remote_retrieve_header( $response, 'Content-Type' ) ) ) {
 		return false;
 	}
 
diff --git src/wp-includes/functions.php src/wp-includes/functions.php
index 946853c361..ab7eb60a77 100644
--- src/wp-includes/functions.php
+++ src/wp-includes/functions.php
@@ -945,8 +945,8 @@ function do_enclose( $content, $post ) {
 
 			$headers = wp_get_http_headers( $url );
 			if ( $headers ) {
-				$len           = isset( $headers['content-length'] ) ? (int) $headers['content-length'] : 0;
-				$type          = isset( $headers['content-type'] ) ? $headers['content-type'] : '';
+				$len           = isset( $headers['Content-Length'] ) ? (int) $headers['Content-Length'] : 0;
+				$type          = isset( $headers['Content-Type'] ) ? $headers['Content-Type'] : '';
 				$allowed_types = array( 'video', 'audio' );
 
 				// Check to see if we can figure out the mime type from the extension.
@@ -978,7 +978,7 @@ function do_enclose( $content, $post ) {
  *
  * @param string $url        URL to retrieve HTTP headers from.
  * @param bool   $deprecated Not Used.
- * @return string|false Headers on success, false on failure.
+ * @return false|Requests_Utility_CaseInsensitiveDictionary Headers on success, false on failure.
  */
 function wp_get_http_headers( $url, $deprecated = false ) {
 	if ( ! empty( $deprecated ) ) {
diff --git src/wp-includes/general-template.php src/wp-includes/general-template.php
index 1a23cb4316..644513ad2b 100644
--- src/wp-includes/general-template.php
+++ src/wp-includes/general-template.php
@@ -755,7 +755,7 @@ function bloginfo( $show = '' ) {
  * - 'admin_email' - Admin email (set in Settings > General)
  * - 'charset' - The "Encoding for pages and feeds"  (set in Settings > Reading)
  * - 'version' - The current WordPress version
- * - 'html_type' - The content-type (default: "text/html"). Themes and plugins
+ * - 'html_type' - The Content-Type (default: "text/html"). Themes and plugins
  *   can override the default value using the {@see 'pre_option_html_type'} filter
  * - 'text_direction' - The text direction determined by the site's language. is_rtl()
  *   should be used instead
diff --git src/wp-includes/pluggable.php src/wp-includes/pluggable.php
index 5ab3eef749..8bdf747aa6 100644
--- src/wp-includes/pluggable.php
+++ src/wp-includes/pluggable.php
@@ -468,7 +468,7 @@ if ( ! function_exists( 'wp_mail' ) ) :
 
 		// Set Content-Type and charset.
 
-		// If we don't have a content-type from the input headers.
+		// If we don't have a Content-Type from the input headers.
 		if ( ! isset( $content_type ) ) {
 			$content_type = 'text/plain';
 		}
diff --git src/wp-includes/rest-api/class-wp-rest-request.php src/wp-includes/rest-api/class-wp-rest-request.php
index b93bd36857..1d48c3f70c 100644
--- src/wp-includes/rest-api/class-wp-rest-request.php
+++ src/wp-includes/rest-api/class-wp-rest-request.php
@@ -291,16 +291,16 @@ class WP_REST_Request implements ArrayAccess {
 	}
 
 	/**
-	 * Retrieves the content-type of the request.
+	 * Retrieves the Content-Type of the request.
 	 *
 	 * @since 4.4.0
 	 *
 	 * @return array|null Map containing 'value' and 'parameters' keys
-	 *                    or null when no valid content-type header was
+	 *                    or null when no valid Content-Type header was
 	 *                    available.
 	 */
 	public function get_content_type() {
-		$value = $this->get_header( 'content-type' );
+		$value = $this->get_header( 'Content-Type' );
 		if ( empty( $value ) ) {
 			return null;
 		}
@@ -325,11 +325,11 @@ class WP_REST_Request implements ArrayAccess {
 	}
 
 	/**
-	 * Checks if the request has specified a JSON content-type.
+	 * Checks if the request has specified a JSON Content-Type.
 	 *
 	 * @since 5.6.0
 	 *
-	 * @return bool True if the content-type header is JSON.
+	 * @return bool True if the Content-Type header is JSON.
 	 */
 	public function is_json_content_type() {
 		$content_type = $this->get_content_type();
@@ -719,7 +719,7 @@ class WP_REST_Request implements ArrayAccess {
 		$this->parsed_body = true;
 
 		/*
-		 * Check that we got URL-encoded. Treat a missing content-type as
+		 * Check that we got URL-encoded. Treat a missing Content-Type as
 		 * URL-encoded for maximum compatibility.
 		 */
 		$content_type = $this->get_content_type();
diff --git src/wp-includes/sitemaps/class-wp-sitemaps-renderer.php src/wp-includes/sitemaps/class-wp-sitemaps-renderer.php
index 11d5c3820a..9f91a10b7c 100644
--- src/wp-includes/sitemaps/class-wp-sitemaps-renderer.php
+++ src/wp-includes/sitemaps/class-wp-sitemaps-renderer.php
@@ -123,7 +123,7 @@ class WP_Sitemaps_Renderer {
 	 * @param array $sitemaps Array of sitemap URLs.
 	 */
 	public function render_index( $sitemaps ) {
-		header( 'Content-type: application/xml; charset=UTF-8' );
+		header( 'Content-Type: application/xml; charset=UTF-8' );
 
 		$this->check_for_simple_xml_availability();
 
@@ -188,7 +188,7 @@ class WP_Sitemaps_Renderer {
 	 * @param array $url_list Array of URLs for a sitemap.
 	 */
 	public function render_sitemap( $url_list ) {
-		header( 'Content-type: application/xml; charset=UTF-8' );
+		header( 'Content-Type: application/xml; charset=UTF-8' );
 
 		$this->check_for_simple_xml_availability();
 
diff --git src/wp-includes/sitemaps/class-wp-sitemaps-stylesheet.php src/wp-includes/sitemaps/class-wp-sitemaps-stylesheet.php
index 8268fcc924..f752a7f55f 100644
--- src/wp-includes/sitemaps/class-wp-sitemaps-stylesheet.php
+++ src/wp-includes/sitemaps/class-wp-sitemaps-stylesheet.php
@@ -22,7 +22,7 @@ class WP_Sitemaps_Stylesheet {
 	 * @param string $type Stylesheet type. Either 'sitemap' or 'index'.
 	 */
 	public function render_stylesheet( $type ) {
-		header( 'Content-type: application/xml; charset=UTF-8' );
+		header( 'Content-Type: application/xml; charset=UTF-8' );
 
 		if ( 'sitemap' === $type ) {
 			// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- All content escaped below.
diff --git src/wp-mail.php src/wp-mail.php
index d3a86352f2..ddc8874e84 100644
--- src/wp-mail.php
+++ src/wp-mail.php
@@ -170,7 +170,7 @@ for ( $i = 1; $i <= $count; $i++ ) {
 		$content = explode( '--' . $boundary, $content );
 		$content = $content[2];
 
-		// Match case-insensitive content-transfer-encoding.
+		// Match case-insensitive Content-Transfer-Encoding.
 		if ( preg_match( '/Content-Transfer-Encoding: quoted-printable/i', $content, $delim ) ) {
 			$content = explode( $delim[0], $content );
 			$content = $content[1];
diff --git tests/phpunit/data/WPHTTP-testcase-redirection-script.php tests/phpunit/data/WPHTTP-testcase-redirection-script.php
index a73e8c6e09..94643d3de9 100644
--- tests/phpunit/data/WPHTTP-testcase-redirection-script.php
+++ tests/phpunit/data/WPHTTP-testcase-redirection-script.php
@@ -36,7 +36,7 @@ if ( isset($_GET['201-location']) ) {
 }
 if ( isset($_GET['header-check']) ) {
 	$out = array();
-	header("Content-type: text/plain");
+	header("Content-Type: text/plain");
 	foreach ( $_SERVER as $key => $value ) {
 		if ( stripos($key, 'http') === 0 ) {
 			$key = strtolower(substr($key, 5));
@@ -63,7 +63,7 @@ if ( isset( $_GET['post-redirect-to-method'] ) ) {
 
 	echo $method;
 	exit;
-	
+
 }
 
 if ( isset( $_GET['location-with-200'] ) ) {
diff --git tests/phpunit/tests/admin/includesFile.php tests/phpunit/tests/admin/includesFile.php
index b598a2d3bd..38d66e8354 100644
--- tests/phpunit/tests/admin/includesFile.php
+++ tests/phpunit/tests/admin/includesFile.php
@@ -156,7 +156,7 @@ class Tests_Admin_IncludesFile extends WP_UnitTestCase {
 				'code' => 200,
 			),
 			'headers'  => array(
-				'content-disposition' => 'attachment; filename="filename-from-content-disposition-header.txt"',
+				'Content-Disposition' => 'attachment; filename="filename-from-content-disposition-header.txt"',
 			),
 		);
 	}
@@ -174,7 +174,7 @@ class Tests_Admin_IncludesFile extends WP_UnitTestCase {
 				'code' => 200,
 			),
 			'headers'  => array(
-				'content-disposition' => 'attachment; filename="../../filename-from-content-disposition-header.txt"',
+				'Content-Disposition' => 'attachment; filename="../../filename-from-content-disposition-header.txt"',
 			),
 		);
 	}
@@ -192,7 +192,7 @@ class Tests_Admin_IncludesFile extends WP_UnitTestCase {
 				'code' => 200,
 			),
 			'headers'  => array(
-				'content-disposition' => 'attachment; filename=filename-from-content-disposition-header.txt',
+				'Content-Disposition' => 'attachment; filename=filename-from-content-disposition-header.txt',
 			),
 		);
 	}
@@ -241,7 +241,7 @@ class Tests_Admin_IncludesFile extends WP_UnitTestCase {
 				'code' => 200,
 			),
 			'headers'  => array(
-				'content-disposition' => 'filename="filename-from-content-disposition-header.txt"',
+				'Content-Disposition' => 'filename="filename-from-content-disposition-header.txt"',
 			),
 		);
 	}
@@ -259,7 +259,7 @@ class Tests_Admin_IncludesFile extends WP_UnitTestCase {
 				'code' => 200,
 			),
 			'headers'  => array(
-				'content-disposition' => 'inline; filename="filename-from-content-disposition-header.txt"',
+				'Content-Disposition' => 'inline; filename="filename-from-content-disposition-header.txt"',
 			),
 		);
 	}
@@ -277,7 +277,7 @@ class Tests_Admin_IncludesFile extends WP_UnitTestCase {
 				'code' => 200,
 			),
 			'headers'  => array(
-				'content-disposition' => 'form-data; name="file"; filename="filename-from-content-disposition-header.txt"',
+				'Content-Disposition' => 'form-data; name="file"; filename="filename-from-content-disposition-header.txt"',
 			),
 		);
 	}
diff --git tests/phpunit/tests/functions/doEnclose.php tests/phpunit/tests/functions/doEnclose.php
index 6c06f35182..71d96edbac 100644
--- tests/phpunit/tests/functions/doEnclose.php
+++ tests/phpunit/tests/functions/doEnclose.php
@@ -265,14 +265,14 @@ class Tests_Functions_DoEnclose extends WP_UnitTestCase {
 		$fake_headers = array(
 			'mp4' => array(
 				'headers' => array(
-					'content-length' => 123,
-					'content-type'   => 'video/mp4',
+					'Content-Length' => 123,
+					'Content-Type'   => 'video/mp4',
 				),
 			),
 			'ogg' => array(
 				'headers' => array(
-					'content-length' => 321,
-					'content-type'   => 'audio/ogg',
+					'Content-Length' => 321,
+					'Content-Type'   => 'audio/ogg',
 				),
 			),
 		);
@@ -289,8 +289,8 @@ class Tests_Functions_DoEnclose extends WP_UnitTestCase {
 		// Fallback header.
 		return array(
 			'headers' => array(
-				'content-length' => 0,
-				'content-type'   => '',
+				'Content-Length' => 0,
+				'Content-Type'   => '',
 			),
 		);
 	}
diff --git tests/phpunit/tests/http/base.php tests/phpunit/tests/http/base.php
index ee42056bb3..9d64f572a8 100644
--- tests/phpunit/tests/http/base.php
+++ tests/phpunit/tests/http/base.php
@@ -283,7 +283,7 @@ abstract class WP_HTTP_UnitTestCase extends WP_UnitTestCase {
 		$this->skipTestOnTimeout( $res );
 		$this->assertNotWPError( $res );
 		$this->assertSame( '', $res['body'] ); // The body should be empty.
-		$this->assertEquals( $size, $res['headers']['content-length'] );   // Check the headers are returned (and the size is the same).
+		$this->assertEquals( $size, $res['headers']['Content-Length'] );   // Check the headers are returned (and the size is the same).
 		$this->assertSame( $size, $filesize ); // Check that the file is written to disk correctly without any extra characters.
 		$this->assertStringStartsWith( get_temp_dir(), $res['filename'] ); // Check it's saving within the temp directory.
 	}
@@ -450,8 +450,8 @@ abstract class WP_HTTP_UnitTestCase extends WP_UnitTestCase {
 		$res = wp_remote_head( $url, array( 'timeout' => 30 ) );
 
 		$this->skipTestOnTimeout( $res );
-		$this->assertIsArray( wp_remote_retrieve_header( $res, 'location' ) );
-		$this->assertCount( 2, wp_remote_retrieve_header( $res, 'location' ) );
+		$this->assertIsArray( wp_remote_retrieve_header( $res, 'Location' ) );
+		$this->assertCount( 2, wp_remote_retrieve_header( $res, 'Location' ) );
 
 		$res = wp_remote_get( $url, array( 'timeout' => 30 ) );
 
diff --git tests/phpunit/tests/http/functions.php tests/phpunit/tests/http/functions.php
index 49854e0053..40e51cb071 100644
--- tests/phpunit/tests/http/functions.php
+++ tests/phpunit/tests/http/functions.php
@@ -20,8 +20,8 @@ class Tests_HTTP_Functions extends WP_UnitTestCase {
 
 		$this->assertIsArray( $response );
 
-		$this->assertSame( 'image/jpeg', $headers['content-type'] );
-		$this->assertSame( '40148', $headers['content-length'] );
+		$this->assertSame( 'image/jpeg', $headers['Content-Type'] );
+		$this->assertSame( '40148', $headers['Content-Length'] );
 		$this->assertSame( 200, wp_remote_retrieve_response_code( $response ) );
 	}
 
@@ -65,8 +65,8 @@ class Tests_HTTP_Functions extends WP_UnitTestCase {
 		$this->assertIsArray( $response );
 
 		// Should return the same headers as a HEAD request.
-		$this->assertSame( 'image/jpeg', $headers['content-type'] );
-		$this->assertSame( '40148', $headers['content-length'] );
+		$this->assertSame( 'image/jpeg', $headers['Content-Type'] );
+		$this->assertSame( '40148', $headers['Content-Length'] );
 		$this->assertSame( 200, wp_remote_retrieve_response_code( $response ) );
 	}
 
@@ -86,8 +86,8 @@ class Tests_HTTP_Functions extends WP_UnitTestCase {
 		$headers = wp_remote_retrieve_headers( $response );
 
 		// Should return the same headers as a HEAD request.
-		$this->assertSame( 'image/jpeg', $headers['content-type'] );
-		$this->assertSame( '40148', $headers['content-length'] );
+		$this->assertSame( 'image/jpeg', $headers['Content-Type'] );
+		$this->assertSame( '40148', $headers['Content-Length'] );
 		$this->assertSame( 200, wp_remote_retrieve_response_code( $response ) );
 	}
 
diff --git tests/phpunit/tests/post/wpAfterInsertPost.php tests/phpunit/tests/post/wpAfterInsertPost.php
index c81ad21387..f184c66e01 100644
--- tests/phpunit/tests/post/wpAfterInsertPost.php
+++ tests/phpunit/tests/post/wpAfterInsertPost.php
@@ -171,7 +171,7 @@ class Tests_Post_wpAfterInsertPost extends WP_UnitTestCase {
 		$post_id = self::$post_id;
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/posts/%d', $post_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( array( 'title' => 'new title' ) );
 		rest_get_server()->dispatch( $request );
 
@@ -188,7 +188,7 @@ class Tests_Post_wpAfterInsertPost extends WP_UnitTestCase {
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', sprintf( '/wp/v2/posts' ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params(
 			array(
 				'title'  => 'new title',
@@ -211,7 +211,7 @@ class Tests_Post_wpAfterInsertPost extends WP_UnitTestCase {
 		$attachment_id = self::$attachment_id;
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/media/%d', $attachment_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( array( 'title' => 'new attachment title' ) );
 		rest_get_server()->dispatch( $request );
 
diff --git tests/phpunit/tests/rest-api/rest-autosaves-controller.php tests/phpunit/tests/rest-api/rest-autosaves-controller.php
index 2a7eacb8b9..4d9ca15531 100644
--- tests/phpunit/tests/rest-api/rest-autosaves-controller.php
+++ tests/phpunit/tests/rest-api/rest-autosaves-controller.php
@@ -311,7 +311,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 		wp_set_current_user( self::$editor_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts/' . self::$post_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 
 		$params = $this->set_post_data(
 			array(
@@ -327,7 +327,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 	public function test_update_item() {
 		wp_set_current_user( self::$editor_id );
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts/' . self::$post_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 
 		$params = $this->set_post_data(
 			array(
@@ -346,7 +346,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 		wp_set_current_user( self::$contributor_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts/' . self::$post_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 
 		$params = $this->set_post_data(
 			array(
@@ -365,7 +365,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 		wp_set_current_user( self::$editor_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts/' . self::$post_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 
 		$current_post = get_post( self::$post_id );
 
@@ -412,7 +412,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts/' . self::$post_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $autosave_data ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -451,7 +451,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts/' . self::$post_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $autosave_data ) );
 
 		$response     = rest_get_server()->dispatch( $request );
@@ -572,7 +572,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 	public function test_update_item_draft_page_with_parent() {
 		wp_set_current_user( self::$editor_id );
 		$request = new WP_REST_Request( 'POST', '/wp/v2/pages/' . self::$child_draft_page_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 
 		$params = $this->set_post_data(
 			array(
@@ -593,7 +593,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 		wp_set_current_user( self::$editor_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/pages/' . self::$draft_page_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 
 		$params = $this->set_post_data(
 			array(
@@ -648,7 +648,7 @@ class WP_Test_REST_Autosaves_Controller extends WP_Test_REST_Post_Type_Controlle
 
 		// Initiate an autosave via the REST API as Gutenberg does.
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts/' . self::$post_id . '/autosaves' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $autosave_data ) );
 
 		$response = rest_get_server()->dispatch( $request );
diff --git tests/phpunit/tests/rest-api/rest-block-renderer-controller.php tests/phpunit/tests/rest-api/rest-block-renderer-controller.php
index 9f2401e893..7f7b468fe4 100644
--- tests/phpunit/tests/rest-api/rest-block-renderer-controller.php
+++ tests/phpunit/tests/rest-api/rest-block-renderer-controller.php
@@ -501,7 +501,7 @@ class REST_Block_Renderer_Controller_Test extends WP_Test_REST_Controller_Testca
 		$attributes       = array( 'some_string' => $string_attribute );
 		$request          = new WP_REST_Request( 'POST', self::$rest_api_route . self::$block_name );
 		$request->set_param( 'context', 'edit' );
-		$request->set_header( 'content-type', 'application/json' );
+		$request->set_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( compact( 'attributes' ) ) );
 		$response = rest_get_server()->dispatch( $request );
 
diff --git tests/phpunit/tests/rest-api/rest-comments-controller.php tests/phpunit/tests/rest-api/rest-comments-controller.php
index eb66ee5d41..0c3b27410c 100644
--- tests/phpunit/tests/rest-api/rest-comments-controller.php
+++ tests/phpunit/tests/rest-api/rest-comments-controller.php
@@ -1133,7 +1133,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1239,7 +1239,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1264,7 +1264,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1287,7 +1287,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1307,7 +1307,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1327,7 +1327,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1347,7 +1347,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1365,7 +1365,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1386,7 +1386,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1413,7 +1413,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1435,7 +1435,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1460,7 +1460,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1481,7 +1481,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1510,7 +1510,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 		$this->assertSame( 201, $response->get_status() );
@@ -1536,7 +1536,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1575,7 +1575,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1598,7 +1598,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1625,7 +1625,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1654,7 +1654,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1679,7 +1679,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1699,7 +1699,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1720,7 +1720,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1744,7 +1744,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1768,8 +1768,8 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
-		$request->add_header( 'user_agent', 'Mozilla/4.0 (compatible; MSIE 5.5; AOL 4.0; Windows 95)' );
+		$request->add_header( 'Content-Type', 'application/json' );
+		$request->add_header( 'User-Agent', 'Mozilla/4.0 (compatible; MSIE 5.5; AOL 4.0; Windows 95)' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1794,7 +1794,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 			'status'       => 'approved',
 		);
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response    = rest_get_server()->dispatch( $request );
 		$data        = $response->get_data();
@@ -1815,7 +1815,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 			'status'       => 'approved',
 		);
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1835,7 +1835,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 		$this->assertErrorResponse( 'rest_comment_invalid_author_ip', $response, 403 );
@@ -1855,7 +1855,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response    = rest_get_server()->dispatch( $request );
 		$data        = $response->get_data();
@@ -1875,7 +1875,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1895,7 +1895,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1915,7 +1915,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1935,7 +1935,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1955,7 +1955,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1976,7 +1976,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -1996,7 +1996,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2023,7 +2023,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2044,7 +2044,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2075,7 +2075,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2093,7 +2093,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2117,7 +2117,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2132,7 +2132,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2156,7 +2156,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2183,7 +2183,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2207,7 +2207,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2231,7 +2231,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2255,7 +2255,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2275,7 +2275,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2296,7 +2296,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 		$this->assertSame( 201, $response->get_status() );
@@ -2319,7 +2319,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2424,7 +2424,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', $comment_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2452,7 +2452,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', $comment_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2474,7 +2474,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2498,7 +2498,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2518,7 +2518,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2537,7 +2537,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2557,7 +2557,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2575,7 +2575,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2592,7 +2592,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2609,7 +2609,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2630,7 +2630,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2646,7 +2646,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2661,7 +2661,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', '/wp/v2/comments/' . REST_TESTS_IMPOSSIBLY_HIGH_NUMBER );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2686,7 +2686,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$hold_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2705,7 +2705,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2735,7 +2735,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', $private_comment_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2794,7 +2794,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2814,7 +2814,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2834,7 +2834,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2853,7 +2853,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2874,7 +2874,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/comments/%d', self::$approved_id ) );
 
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
diff --git tests/phpunit/tests/rest-api/rest-posts-controller.php tests/phpunit/tests/rest-api/rest-posts-controller.php
index 5fa7e3cf98..eccb1f24c6 100644
--- tests/phpunit/tests/rest-api/rest-posts-controller.php
+++ tests/phpunit/tests/rest-api/rest-posts-controller.php
@@ -2301,7 +2301,7 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		wp_set_current_user( self::$editor_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_post_data();
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
@@ -2492,7 +2492,7 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		wp_set_current_user( self::$editor_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/posts' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$params = $this->set_post_data();
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
@@ -3123,7 +3123,7 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		wp_set_current_user( self::$editor_id );
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/posts/%d', self::$post_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_post_data();
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
@@ -3161,7 +3161,7 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		wp_set_current_user( self::$editor_id );
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/posts/%d', self::$post_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$params = $this->set_post_data();
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
@@ -3195,7 +3195,7 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		$future_date = '2919-07-29T18:00:00';
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/posts/%d', $post_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$params = $this->set_post_data(
 			array(
 				'date_gmt' => $future_date,
@@ -3217,7 +3217,7 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 
 		// Update post with a blank field (date or date_gmt).
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/posts/%d', $post_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$params = $this->set_post_data(
 			array(
 				'date_gmt' => null,
@@ -3244,7 +3244,7 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		wp_set_current_user( self::$editor_id );
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/posts/%d', self::$post_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$params = $this->set_raw_post_data();
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
diff --git tests/phpunit/tests/rest-api/rest-request.php tests/phpunit/tests/rest-api/rest-request.php
index 509c754f65..cf1f3ac739 100644
--- tests/phpunit/tests/rest-api/rest-request.php
+++ tests/phpunit/tests/rest-api/rest-request.php
@@ -203,7 +203,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 	 * @ticket 49404
 	 * @dataProvider alternate_json_content_type_provider
 	 *
-	 * @param string $content_type The content-type header.
+	 * @param string $content_type The Content-Type header.
 	 * @param string $source       The source value.
 	 * @param bool   $accept_json  The accept_json value.
 	 */
@@ -234,7 +234,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 	 * @ticket 49404
 	 * @dataProvider is_json_content_type_provider
 	 *
-	 * @param string $content_type The content-type header.
+	 * @param string $content_type The Content-Type header.
 	 * @param bool   $is_json      The is_json value.
 	 */
 	public function test_is_json_content_type( $content_type, $is_json ) {
@@ -242,7 +242,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 
 		$this->request->set_header( 'Content-Type', $content_type );
 
-		// Check for JSON content-type.
+		// Check for JSON Content-Type.
 		$this->assertSame( $is_json, $this->request->is_json_content_type() );
 	}
 
@@ -361,7 +361,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 		);
 
 		$this->request->set_method( 'PUT' );
-		$this->request->add_header( 'content-type', 'application/json' );
+		$this->request->add_header( 'Content-Type', 'application/json' );
 		$this->request->set_body( wp_json_encode( $data ) );
 
 		foreach ( $data as $key => $expected_value ) {
@@ -383,7 +383,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 		);
 
 		$this->request->set_method( 'POST' );
-		$this->request->add_header( 'content-type', 'application/json' );
+		$this->request->add_header( 'Content-Type', 'application/json' );
 		$this->request->set_body( wp_json_encode( $data ) );
 
 		foreach ( $data as $key => $expected_value ) {
@@ -864,7 +864,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 
 	public function test_set_param_follows_parameter_order() {
 		$request = new WP_REST_Request();
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_method( 'POST' );
 		$request->set_body(
 			wp_json_encode(
@@ -892,7 +892,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 	 */
 	public function test_set_param_updates_param_in_json_and_query() {
 		$request = new WP_REST_Request();
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_method( 'POST' );
 		$request->set_body(
 			wp_json_encode(
@@ -919,7 +919,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 	 */
 	public function test_set_param_updates_param_if_already_exists_in_query() {
 		$request = new WP_REST_Request();
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_method( 'POST' );
 		$request->set_body(
 			wp_json_encode(
@@ -953,7 +953,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 	 */
 	public function test_set_param_to_null_updates_param_in_json_and_query() {
 		$request = new WP_REST_Request();
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_method( 'POST' );
 		$request->set_body(
 			wp_json_encode(
@@ -980,7 +980,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 	 */
 	public function test_set_param_from_null_updates_param_in_json_and_query_with_null() {
 		$request = new WP_REST_Request();
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_method( 'POST' );
 		$request->set_body(
 			wp_json_encode(
@@ -1007,7 +1007,7 @@ class Tests_REST_Request extends WP_UnitTestCase {
 	 */
 	public function test_set_param_with_invalid_json() {
 		$request = new WP_REST_Request();
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_method( 'POST' );
 		$request->set_body( '' );
 		$request->set_param( 'param', 'value' );
diff --git tests/phpunit/tests/rest-api/rest-settings-controller.php tests/phpunit/tests/rest-api/rest-settings-controller.php
index 60a5f28c8d..a2bbcb79c6 100644
--- tests/phpunit/tests/rest-api/rest-settings-controller.php
+++ tests/phpunit/tests/rest-api/rest-settings-controller.php
@@ -773,7 +773,7 @@ class WP_Test_REST_Settings_Controller extends WP_Test_REST_Controller_Testcase
 			),
 		);
 		$request = new WP_REST_Request( 'PUT', '/wp/v2/settings' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $data ) );
 
 		$response = rest_do_request( $request );
diff --git tests/phpunit/tests/rest-api/rest-users-controller.php tests/phpunit/tests/rest-api/rest-users-controller.php
index 849b0e5464..1f875df16b 100644
--- tests/phpunit/tests/rest-api/rest-users-controller.php
+++ tests/phpunit/tests/rest-api/rest-users-controller.php
@@ -1230,7 +1230,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1263,7 +1263,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		}
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 		$this->assertErrorResponse( 'rest_invalid_param', $response, 400 );
@@ -1308,7 +1308,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1338,7 +1338,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 		$data     = $response->get_data();
@@ -1369,7 +1369,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		add_filter( 'can_add_user_to_blog', '__return_false' );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 		$this->assertErrorResponse( 'user_cannot_be_added', $response );
@@ -1392,7 +1392,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		switch_to_blog( self::$site );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 		$data     = $response->get_data();
@@ -1422,7 +1422,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 		$data     = $response->get_data();
@@ -1431,7 +1431,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		switch_to_blog( self::$site );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$switched_response = rest_get_server()->dispatch( $request );
 
@@ -1470,7 +1470,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1487,7 +1487,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1507,7 +1507,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1526,7 +1526,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1546,7 +1546,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/users' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1579,7 +1579,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		$_POST['locale']     = 'de_DE';
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/users/%d', $user_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $_POST );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -1826,7 +1826,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		$pw_before = $userdata->user_pass;
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/users/%d', $user_id ) );
-		$request->add_header( 'content-type', 'application/json' );
+		$request->add_header( 'Content-Type', 'application/json' );
 		$request->set_body( wp_json_encode( $params ) );
 
 		$response = rest_get_server()->dispatch( $request );
@@ -2008,14 +2008,14 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/users/%d', self::$user ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
 		$this->assertErrorResponse( 'rest_cannot_edit', $response, 403 );
 
 		$request = new WP_REST_Request( 'PUT', '/wp/v2/users/me' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2035,7 +2035,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		);
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/users/%d', self::$editor ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
 
@@ -2881,7 +2881,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		wp_set_current_user( self::$user );
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/users/%d', $user_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( array( 'first_name' => 'New Name' ) );
 		$response = rest_get_server()->dispatch( $request );
 		$this->assertErrorResponse( 'rest_user_invalid_id', $response, 404 );
@@ -2903,7 +2903,7 @@ class WP_Test_REST_Users_Controller extends WP_Test_REST_Controller_Testcase {
 		wp_set_current_user( self::$superadmin );
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/users/%d', $user_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$request->set_body_params( array( 'first_name' => 'New Name' ) );
 		$response = rest_get_server()->dispatch( $request );
 		$this->assertErrorResponse( 'rest_user_invalid_id', $response, 404 );
diff --git tests/phpunit/tests/rest-api/rest-widgets-controller.php tests/phpunit/tests/rest-api/rest-widgets-controller.php
index 28ac8bc6e3..dff2289278 100644
--- tests/phpunit/tests/rest-api/rest-widgets-controller.php
+++ tests/phpunit/tests/rest-api/rest-widgets-controller.php
@@ -412,7 +412,7 @@ class WP_Test_REST_Widgets_Controller extends WP_Test_REST_Controller_Testcase {
 
 	public function mocked_rss_response() {
 		$single_value_headers = array(
-			'content-type' => 'application/rss+xml; charset=UTF-8',
+			'Content-Type' => 'application/rss+xml; charset=UTF-8',
 			'link'         => '<https://wordpress.org/news/wp-json/>; rel="https://api.w.org/"',
 		);
 
diff --git tests/phpunit/tests/rest-api/wpRestMenuItemsController.php tests/phpunit/tests/rest-api/wpRestMenuItemsController.php
index 9ef5b87d99..ecbc491256 100644
--- tests/phpunit/tests/rest-api/wpRestMenuItemsController.php
+++ tests/phpunit/tests/rest-api/wpRestMenuItemsController.php
@@ -318,7 +318,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data();
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
@@ -334,7 +334,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'menus' => array( 123, 456 ),
@@ -354,7 +354,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'type'  => 'taxonomy',
@@ -377,7 +377,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		$actual      = array();
 		for ( $i = 1; $i < 5; $i ++ ) {
 			$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-			$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+			$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 			$params = $this->set_menu_item_data(
 				array(
 					'menu_order' => $i,
@@ -404,7 +404,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		$new_menu_id = wp_create_nav_menu( rand_str() );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'menu_order' => 0,
@@ -416,7 +416,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		$this->assertErrorResponse( 'rest_invalid_param', $response, 400 );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'menu_order' => 1,
@@ -436,7 +436,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 		$new_menu_id = wp_create_nav_menu( rand_str() );
 		$request     = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'menu_order' => 'ddddd',
@@ -456,7 +456,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 		$new_menu_id = wp_create_nav_menu( rand_str() );
 		$request     = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'menu_order' => -9,
@@ -476,7 +476,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 		wp_create_nav_menu( rand_str() );
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'parent' => -9,
@@ -494,7 +494,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 	public function test_create_item_invalid_menu() {
 		wp_set_current_user( self::$admin_id );
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'menus' => -9,
@@ -513,7 +513,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'type'  => 'post_type',
@@ -533,7 +533,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'type'             => 'post_type_archive',
@@ -553,7 +553,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'type'  => 'custom',
@@ -573,7 +573,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'type' => 'custom',
@@ -593,7 +593,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'type' => 'custom',
@@ -615,7 +615,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/menu-items/%d', $this->menu_item_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'xfn' => array( 'test1', 'test2', 'test3' ),
@@ -648,7 +648,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		$good_data = array( 'test1', 'test2', 'test3', 'test4' );
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/menu-items/%d', $this->menu_item_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data(
 			array(
 				'xfn' => $bad_data,
@@ -680,7 +680,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		$post_id = self::factory()->post->create();
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/menu-items/%d', $post_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$params = $this->set_menu_item_data();
 		$request->set_body_params( $params );
 		$response = rest_get_server()->dispatch( $request );
@@ -1021,7 +1021,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'POST', '/wp/v2/menu-items' );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$parameters = $this->set_menu_item_data(
 			array(
 				'title' => 'Some \\\'title',
@@ -1043,7 +1043,7 @@ class Tests_REST_WpRestMenuItemsController extends WP_Test_REST_Post_Type_Contro
 		wp_set_current_user( self::$admin_id );
 
 		$request = new WP_REST_Request( 'PUT', sprintf( '/wp/v2/menu-items/%d', $this->menu_item_id ) );
-		$request->add_header( 'content-type', 'application/x-www-form-urlencoded' );
+		$request->add_header( 'Content-Type', 'application/x-www-form-urlencoded' );
 		$title  = 'Some \\\'title';
 		$params = $this->set_menu_item_data(
 			array(
diff --git tests/phpunit/tests/widgets/wpWidgetRss.php tests/phpunit/tests/widgets/wpWidgetRss.php
index 495869f475..e5f65c6d15 100644
--- tests/phpunit/tests/widgets/wpWidgetRss.php
+++ tests/phpunit/tests/widgets/wpWidgetRss.php
@@ -101,7 +101,7 @@ class Tests_Widgets_wpWidgetRss extends WP_UnitTestCase {
 
 	public function mocked_rss_response() {
 		$single_value_headers = array(
-			'content-type' => 'application/rss+xml; charset=UTF-8',
+			'Content-Type' => 'application/rss+xml; charset=UTF-8',
 			'link'         => '<https://wordpress.org/news/wp-json/>; rel="https://api.w.org/"',
 		);
 
