wp_validate_redirect( string $location, string $default = '' )

function wp_validate_redirect($location, $default = '') {
	$location = wp_sanitize_redirect( trim( $location, " \t\n\r\0\x08\x0B" ) );
	// browsers will assume 'http' is your protocol, and will obey a redirect to a URL starting with '//'
	if ( substr($location, 0, 2) == '//' )
		$location = 'http:' . $location;

	// In php 5 parse_url may fail if the URL query part contains http://, bug https://core.trac.wordpress.org/ticket/38143
	$test = ( $cut = strpos($location, '?') ) ? substr( $location, 0, $cut ) : $location;

	// @-operator is used to prevent possible warnings in PHP < 5.3.3.
	$lp = @parse_url($test);

	// Give up if malformed URL
	if ( false === $lp )
		return $default;

	// Allow only http and https schemes. No data:, etc.
	if ( isset($lp['scheme']) && !('http' == $lp['scheme'] || 'https' == $lp['scheme']) )
		return $default;

	if ( ! isset( $lp['host'] ) && ! empty( $lp['path'] ) && '/' !== $lp['path'][0] ) {
		$path = '';
		if ( ! empty( $_SERVER['REQUEST_URI'] ) ) {
			$path = dirname( parse_url( 'http://placeholder' . $_SERVER['REQUEST_URI'], PHP_URL_PATH ) . '?' );
		}
		$location = '/' . ltrim( $path . '/', '/' ) . $location;
	}

	// Reject if certain components are set but host is not. This catches urls like https:host.com for which parse_url does not set the host field.
	if ( ! isset( $lp['host'] ) && ( isset( $lp['scheme'] ) || isset( $lp['user'] ) || isset( $lp['pass'] ) || isset( $lp['port'] ) ) ) {
		return $default;
	}

	// Reject malformed components parse_url() can return on odd inputs.
	foreach ( array( 'user', 'pass', 'host' ) as $component ) {
		if ( isset( $lp[ $component ] ) && strpbrk( $lp[ $component ], ':/?#@' ) ) {
			return $default;
		}
	}

	$wpp = parse_url(home_url());

	/**
	 * Filters the whitelist of hosts to redirect to.
	 *
	 * @since WP-2.3.0
	 *
	 * @param array       $hosts An array of allowed hosts.
	 * @param bool|string $host  The parsed host; empty if not isset.
	 */
	$allowed_hosts = (array) apply_filters( 'allowed_redirect_hosts', array($wpp['host']), isset($lp['host']) ? $lp['host'] : '' );

	if ( isset($lp['host']) && ( !in_array($lp['host'], $allowed_hosts) && $lp['host'] != strtolower($wpp['host'])) )
		$location = $default;

	return $location;
}