protected static function _getBackend()
    {
        if (!isset(self::$_backend)) {
            if (extension_loaded('intl')) {
                /* Only available in PHP > 5.4.0 */
                self::$_backend = defined('INTL_IDNA_VARIANT_UTS46')
                    ? 'INTL_UTS46'
                    : 'INTL';
            } else {
                self::$_backend = new Horde_Idna_Punycode();
            }
        }

        return self::$_backend;
    }