Source

File: wp-includes/http.php

function is_allowed_http_origin( $origin = null ) {
	$origin_arg = $origin;

	if ( null === $origin ) {
		$origin = get_http_origin();
	}

	if ( $origin && ! in_array( $origin, get_allowed_http_origins(), true ) ) {
		$origin = '';
	}

	/**
	 * Change the allowed HTTP origin result.
	 *
	 * @since 3.4.0
	 *
	 * @param string $origin     Origin URL if allowed, empty string if not.
	 * @param string $origin_arg Original origin string passed into is_allowed_http_origin function.
	 */
	return apply_filters( 'allowed_http_origin', $origin, $origin_arg );
}