private static function isAllowedScheme($scheme)
    {
        if (preg_match(AbstractRequest::HTTP, $scheme) === 1) {
            return 'http';
        } elseif (preg_match(AbstractRequest::FTP, $scheme) === 1) {
            return 'ftp';
        } elseif (preg_match(AbstractRequest::SSH, $scheme) === 1) {
            return 'ssh';
        } else {
            return false;
        }
    }