diff --git src/wp-includes/functions.php src/wp-includes/functions.php
index dce925a..ffbb196 100644
--- src/wp-includes/functions.php
+++ src/wp-includes/functions.php
@@ -4740,7 +4740,9 @@ function reset_mbstring_encoding() {
 }
 
 /**
- * Alternative to filter_var( $var, FILTER_VALIDATE_BOOLEAN ).
+ * Filter/validate a variable as a boolean.
+ *
+ * Alternative to `filter_var( $var, FILTER_VALIDATE_BOOLEAN )`.
  *
  * @since 4.0.0
  *
@@ -4752,7 +4754,7 @@ function wp_validate_boolean( $var ) {
 		return $var;
 	}
 
-	if ( 'false' === $var ) {
+	if ( 'false' === strtolower( $var ) ) {
 		return false;
 	}