wp_validate_redirect

	/**
	 * Validates a URL for use in a redirect.
	 *
	 * Checks whether the $location is using an allowed host, if it has an absolute
	 * path. A plugin can therefore set or remove allowed host(s) to or from the
	 * list.
	 *
	 * If the host is not allowed, then the redirect is to $default supplied
	 *
	 * @since 2.8.1
	 *
	 * @param string $location The redirect to validate
	 * @param string $default  The value to return if $location is not allowed
	 * @return string redirect-sanitized URL
	 */
	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://'.
		// See https://bugs.php.net/bug.php?id=38143
		$cut  = strpos( $location, '?' );
		$test = $cut ? substr( $location, 0, $cut ) : $location;

		$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 ) . '?' );
				$path = wp_normalize_path( $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 list of allowed hosts to redirect to.