diff --git src/wp-admin/includes/ajax-actions.php src/wp-admin/includes/ajax-actions.php
index 83b682634b..514493409b 100644
--- src/wp-admin/includes/ajax-actions.php
+++ src/wp-admin/includes/ajax-actions.php
@@ -119,7 +119,7 @@ function wp_ajax_ajax_tag_search() {
 		wp_die( -1 );
 	}
 
-	$search = wp_unslash( $_GET['q'] );
+	$search = isset( $_GET['q'] ) ? wp_unslash( $_GET['q'] ) : '';
 
 	$comma = _x( ',', 'tag delimiter' );
 	if ( ',' !== $comma ) {
diff --git src/wp-admin/includes/bookmark.php src/wp-admin/includes/bookmark.php
index 03e8b1201c..7849027406 100644
--- src/wp-admin/includes/bookmark.php
+++ src/wp-admin/includes/bookmark.php
@@ -34,14 +34,17 @@ function edit_link( $link_id = 0 ) {
 		);
 	}
 
-	$_POST['link_url']   = esc_url( $_POST['link_url'] );
-	$_POST['link_name']  = esc_html( $_POST['link_name'] );
-	$_POST['link_image'] = esc_html( $_POST['link_image'] );
-	$_POST['link_rss']   = esc_url( $_POST['link_rss'] );
+	$_POST['link_url']   = isset( $_POST['link_url'] ) ? esc_url( $_POST['link_url'] ) : '';
+	$_POST['link_name']  = isset( $_POST['link_name'] ) ? esc_html( $_POST['link_name'] ) : '';
+	$_POST['link_image'] = isset( $_POST['link_image'] ) ? esc_html( $_POST['link_image'] ) : '';
+	$_POST['link_rss']   = isset( $_POST['link_rss'] ) ? esc_url( $_POST['link_rss'] ) : '';
+
+	// If the link is empty, return 0.
 	if ( ! isset( $_POST['link_visible'] ) || 'N' !== $_POST['link_visible'] ) {
 		$_POST['link_visible'] = 'Y';
 	}
 
+	// If the link name is empty, set it to the URL.
 	if ( ! empty( $link_id ) ) {
 		$_POST['link_id'] = $link_id;
 		return wp_update_link( $_POST );
diff --git src/wp-admin/includes/class-wp-terms-list-table.php src/wp-admin/includes/class-wp-terms-list-table.php
index c7f34631a9..be61bcc630 100644
--- src/wp-admin/includes/class-wp-terms-list-table.php
+++ src/wp-admin/includes/class-wp-terms-list-table.php
@@ -402,7 +402,7 @@ class WP_Terms_List_Table extends WP_List_Table {
 
 		$qe_data = get_term( $tag->term_id, $taxonomy, OBJECT, 'edit' );
 
-		$uri = wp_doing_ajax() ? wp_get_referer() : $_SERVER['REQUEST_URI'];
+		$uri = wp_doing_ajax() ? wp_get_referer() : ( isset( $_SERVER['REQUEST_URI'] ) ? $_SERVER['REQUEST_URI'] : '' );
 
 		$edit_link = get_edit_term_link( $tag, $taxonomy, $this->screen->post_type );
 
@@ -473,7 +473,7 @@ class WP_Terms_List_Table extends WP_List_Table {
 		$tag = $item;
 
 		$taxonomy = $this->screen->taxonomy;
-		$uri      = wp_doing_ajax() ? wp_get_referer() : $_SERVER['REQUEST_URI'];
+		$uri      = wp_doing_ajax() ? wp_get_referer() : ( isset( $_SERVER['REQUEST_URI'] ) ? $_SERVER['REQUEST_URI'] : '' );
 
 		$actions = array();
 
diff --git src/wp-admin/includes/comment.php src/wp-admin/includes/comment.php
index ffec90c81e..00a5da2da2 100644
--- src/wp-admin/includes/comment.php
+++ src/wp-admin/includes/comment.php
@@ -82,12 +82,12 @@ function edit_comment() {
 	}
 
 	if ( ! empty( $_POST['edit_date'] ) ) {
-		$aa = $_POST['aa'];
-		$mm = $_POST['mm'];
-		$jj = $_POST['jj'];
-		$hh = $_POST['hh'];
-		$mn = $_POST['mn'];
-		$ss = $_POST['ss'];
+		$aa = isset( $_POST['aa'] ) ? $_POST['aa'] : '';
+		$mm = isset( $_POST['mm'] ) ? $_POST['mm'] : '';
+		$jj = isset( $_POST['jj'] ) ? $_POST['jj'] : '';
+		$hh = isset( $_POST['hh'] ) ? $_POST['hh'] : '';
+		$mn = isset( $_POST['mn'] ) ? $_POST['mn'] : '';
+		$ss = isset( $_POST['ss'] ) ? $_POST['ss'] : '';
 		$jj = ( $jj > 31 ) ? 31 : $jj;
 		$hh = ( $hh > 23 ) ? $hh - 24 : $hh;
 		$mn = ( $mn > 59 ) ? $mn - 60 : $mn;
diff --git src/wp-admin/includes/dashboard.php src/wp-admin/includes/dashboard.php
index b198325f27..08f5bd79c1 100644
--- src/wp-admin/includes/dashboard.php
+++ src/wp-admin/includes/dashboard.php
@@ -149,7 +149,7 @@ function wp_dashboard_setup() {
 		wp_add_dashboard_widget( $widget_id, $name, $wp_registered_widgets[ $widget_id ]['callback'], $wp_registered_widget_controls[ $widget_id ]['callback'] );
 	}
 
-	if ( 'POST' === $_SERVER['REQUEST_METHOD'] && isset( $_POST['widget_id'] ) ) {
+	if ( ( isset( $_SERVER['REQUEST_METHOD'] ) && 'POST' === $_SERVER['REQUEST_METHOD'] ) && isset( $_POST['widget_id'] ) ) {
 		check_admin_referer( 'edit-dashboard-widget_' . $_POST['widget_id'], 'dashboard-widget-nonce' );
 		ob_start(); // Hack - but the same hack wp-admin/widgets.php uses.
 		wp_dashboard_trigger_widget_control( $_POST['widget_id'] );
@@ -1262,7 +1262,7 @@ function wp_dashboard_rss_control( $widget_id, $form_inputs = array() ) {
 
 	$widget_options[ $widget_id ]['number'] = $number;
 
-	if ( 'POST' === $_SERVER['REQUEST_METHOD'] && isset( $_POST['widget-rss'][ $number ] ) ) {
+	if ( ( isset( $_SERVER['REQUEST_METHOD'] ) && 'POST' === $_SERVER['REQUEST_METHOD'] ) && isset( $_POST['widget-rss'][ $number ] ) ) {
 		$_POST['widget-rss'][ $number ]         = wp_unslash( $_POST['widget-rss'][ $number ] );
 		$widget_options[ $widget_id ]           = wp_widget_rss_process( $_POST['widget-rss'][ $number ] );
 		$widget_options[ $widget_id ]['number'] = $number;