private function __getQueryString($queryAr = array(), $prefixIfNotEmpty = '')
    {
        $queryAr = array_filter($queryAr);
        array_walk($queryAr, function (&$val, $key) {
            $val = urlencode($key) . '=' . urlencode($val);
        });

        return $queryAr ? $prefixIfNotEmpty . join('&', $queryAr) : '';
    }