diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 3d9cedb7ecdda..14cc9546528e6 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -7543,9 +7543,10 @@ function _device_can_upload() {
 }
 
 /**
- * Tests if a given path is a stream URL
+ * Tests if a given path is a stream URL.
  *
  * @since 3.5.0
+ * @since 7.2.0 A lowercased scheme is accepted as a fallback, matching PHP.
  *
  * @param string $path The resource path or URL.
  * @return bool True if the path is a stream URL.
@@ -7558,9 +7559,15 @@ function wp_is_stream( $path ) {
 		return false;
 	}
 
-	$stream = substr( $path, 0, $scheme_separator );
+	$stream   = substr( $path, 0, $scheme_separator );
+	$wrappers = stream_get_wrappers();
 
-	return in_array( $stream, stream_get_wrappers(), true );
+	/*
+	 * PHP looks the scheme up as given, then retries once with it lowercased.
+	 * This is not a case-insensitive match: a wrapper registered as `MyStream`
+	 * is not reachable as `mystream`.
+	 */
+	return in_array( $stream, $wrappers, true ) || in_array( strtolower( $stream ), $wrappers, true );
 }
 
 /**
diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index 608b31f058d99..5b5660df2439a 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -2121,6 +2121,8 @@ public function data_validate_file() {
 	/**
 	 * Test stream URL validation.
 	 *
+	 * @ticket 65870
+	 *
 	 * @dataProvider data_wp_is_stream
 	 *
 	 * @param string $path     The resource path or URL.
@@ -2147,17 +2149,82 @@ public function test_wp_is_stream( $path, $expected ) {
 	public function data_wp_is_stream() {
 		return array(
 			// Legitimate stream examples.
-			array( 'http://example.com', true ),
-			array( 'https://example.com', true ),
-			array( 'ftp://example.com', true ),
-			array( 'file:///path/to/some/file', true ),
-			array( 'php://some/php/file.php', true ),
+			'HTTP URL'                   => array( 'http://example.com', true ),
+			'HTTPS URL'                  => array( 'https://example.com', true ),
+			'FTP URL'                    => array( 'ftp://example.com', true ),
+			'file URL'                   => array( 'file:///path/to/some/file', true ),
+			'uppercase file scheme'      => array( 'FILE:///path/to/some/file', true ),
+			'PHP stream URL'             => array( 'php://some/php/file.php', true ),
 
 			// Non-stream examples.
-			array( 'fakestream://foo/bar/baz', false ),
-			array( '../../some/relative/path', false ),
-			array( 'some/other/relative/path', false ),
-			array( '/leading/relative/path', false ),
+			'unregistered stream scheme' => array( 'fakestream://foo/bar/baz', false ),
+			'parent-relative path'       => array( '../../some/relative/path', false ),
+			'relative path'              => array( 'some/other/relative/path', false ),
+			'absolute path'              => array( '/leading/relative/path', false ),
+		);
+	}
+
+	/**
+	 * Tests scheme case matching against PHP's wrapper selection.
+	 *
+	 * @ticket 65870
+	 *
+	 * @dataProvider data_wp_is_stream_matches_php_case_matching
+	 *
+	 * @param string $scheme   The registered scheme.
+	 * @param string $path     The path to open.
+	 * @param bool   $expected Whether PHP should select the registered wrapper.
+	 */
+	public function test_wp_is_stream_matches_php_case_matching( $scheme, $path, $expected ) {
+		$wrapper = new class() {
+			public $context;
+			public static $invoked = false;
+
+			public function stream_open( $path, $mode, $options, &$opened_path ) {
+				self::$invoked = true;
+				return false;
+			}
+		};
+
+		$wrapper::$invoked = false;
+		$registered        = false;
+		$handle            = false;
+
+		try {
+			$registered = stream_wrapper_register( $scheme, get_class( $wrapper ) );
+			$this->assertTrue( $registered );
+
+			// Record selection without requiring the wrapper to parse or open the URL.
+			$handle = @fopen( $path, 'r' );
+
+			$this->assertSame( $expected, $wrapper::$invoked, 'PHP did not select the expected wrapper.' );
+			$this->assertSame( $wrapper::$invoked, wp_is_stream( $path ), 'WordPress and PHP disagree on scheme case matching.' );
+		} finally {
+			if ( is_resource( $handle ) ) {
+				fclose( $handle );
+			}
+			if ( $registered ) {
+				stream_wrapper_unregister( $scheme );
+			}
+		}
+	}
+
+	/**
+	 * Data provider for scheme case matching against PHP.
+	 *
+	 * @return array[]
+	 */
+	public function data_wp_is_stream_matches_php_case_matching() {
+		return array(
+			'lowercase scheme'                  => array( 'wpteststream', 'wpteststream://bucket/file', true ),
+			'lowercase registration mixed case' => array( 'wpteststream', 'wpTestStream://bucket/file', true ),
+			'lowercase registration uppercase'  => array( 'wpteststream', 'WPTESTSTREAM://bucket/file', true ),
+			'mixed case exact match'            => array( 'wpTestStream', 'wpTestStream://bucket/file', true ),
+			'mixed case lowercased'             => array( 'wpTestStream', 'wpteststream://bucket/file', false ),
+			'mixed case uppercased'             => array( 'wpTestStream', 'WPTESTSTREAM://bucket/file', false ),
+			'mixed case with different casing'  => array( 'wpTestStream', 'wptestSTREAM://bucket/file', false ),
+			'uppercase exact match'             => array( 'WPTESTSTREAM', 'WPTESTSTREAM://bucket/file', true ),
+			'uppercase lowercased'              => array( 'WPTESTSTREAM', 'wpteststream://bucket/file', false ),
 		);
 	}