private function curl($query_string)
    {
        $curl = curl_init();

        curl_setopt($curl, CURLOPT_POSTFIELDS, $query_string);
        curl_setopt($curl, CURLOPT_VERBOSE, 1);
        curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, 2);
        curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, 1);
        curl_setopt($curl, CURLOPT_CAINFO, $this->cert_path); // because of Self-Signed certificate at payment server.
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($curl, CURLOPT_SSLCERT, $this->cert_path);
        curl_setopt($curl, CURLOPT_SSLKEY, $this->cert_path);
        curl_setopt($curl, CURLOPT_SSLKEYPASSWD, $this->cert_pass);
        curl_setopt($curl, CURLOPT_URL, $this->submit_url);

        $result = curl_exec($curl);

        return $result;
    }