public function getQueryString()
    {
        $query_string = '';
        if (count($this->query_string) > 0) {
            $query_string .= http_build_query($this->query_string);
        }
        if ($this->method != 'POST') {
            if (count($this->data) > 0) {
                $query_string .= http_build_query($this->data);
            }
        }
        if (mb_strlen($query_string) > 0) {
            $query_string = (mb_strpos($this->url, '?') ? '&' : '?') . $query_string;
        }

        return $query_string;
    }