Themes: Add a hook to filter theme header image URL.

This changeset introduces the `get_header_image` filter, which can be used to modify header image URL returned by `get_header_image()`, in themes that support the Header Image feature. 

Props hztyfoon, audrasjb, mukesh27, SergeyBiryukov, costdev.
Fixes #56180.


git-svn-id: https://develop.svn.wordpress.org/trunk@53741 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -1179,6 +1179,20 @@ function get_header_image() {
 		$url = get_random_header_image();
 	}
 
+	/**
+	 * Filters the header image URL.
+	 *
+	 * @since 6.1.0
+	 *
+	 * @param string $url Header image URL.
+	 */
+	$url = apply_filters( 'get_header_image', $url );
+
+	if ( ! is_string( $url ) ) {
+		return false;
+	}
+
+	$url = trim( $url );
 	return sanitize_url( set_url_scheme( $url ) );
 }
 

tests/phpunit/tests/theme/customHeader.php

@@ -81,6 +81,59 @@ class Tests_Theme_CustomHeader extends WP_UnitTestCase {
 		$this->assertFalse( $image );
 	}
 
+	/**
+	 * Tests the "get_header_image" filter.
+	 *
+	 * @ticket 56180
+	 *
+	 * @covers get_header_image
+	 *
+	 * @dataProvider data_filter_header_image
+	 *
+	 * @param mixed  $header_image The header image.
+	 * @param string $expected     The expected return value from get_header_image().
+	 */
+	public function test_filter_header_image( $header_image, $expected ) {
+		add_filter(
+			'get_header_image',
+			static function() use ( $header_image ) {
+				return $header_image;
+			}
+		);
+
+		$this->assertSame( $expected, get_header_image() );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_filter_header_image() {
+		return array(
+			'an image url'         => array(
+				'header_image' => 'http://example.org/image.png',
+				'expected'     => 'http://example.org/image.png',
+			),
+			'an empty string'      => array(
+				'header_image' => '',
+				'expected'     => '',
+			),
+			'a string with spaces' => array(
+				'header_image' => ' ',
+				'expected'     => '',
+			),
+			'null'                 => array(
+				'header_image' => null,
+				'expected'     => false,
+			),
+			'false'                => array(
+				'header_image' => false,
+				'expected'     => false,
+			),
+		);
+	}
+
 	public function test_get_header_image_tag_without_registered_default_image() {
 		$this->add_theme_support();
 		$html = get_header_image_tag();