private function setPostFields(array $post_data)
    {
        $pd = [];
        foreach ($post_data as $k => $v) {
            $pd[] = "$k=$v";
        }
        $pd = implode("&", $pd);
        \Unirest\Request::curlOpt(CURLOPT_POST, true);
        \Unirest\Request::curlOpt(CURLOPT_POSTFIELDS, $pd);
        \Unirest\Request::curlOpt(CURLOPT_HTTPHEADER, ['Content-Type: application/x-www-form-urlencoded']);
    }