captcha ที่ใช้ session ยังปลอดภัยไหมครับ มีวิธีอื่นอีกไหม

// credit : Konstantin Kolodnitsky
class StopForumSpam{
    function checkIP($ip) {
        $obj = json_decode($this->getResponse("http://stopforumspam.com/api?ip=" . $ip . "&f=json"));
        if (isset($obj->ip->frequency) && $obj->ip->frequency != 0) {
            return true;
        } else {
            return false;
        }
    }

    function checkEmail($email) {
        $host = explode('@', $email);
        if (!filter_var($email, FILTER_VALIDATE_EMAIL) || !$this->checkHostName($host[1]))
            return false;
        $obj = json_decode($this->getResponse("http://stopforumspam.com/api?email=" . $email . "&f=json"));
        if (isset($obj->email->frequency) && $obj->email->frequency != 0) {
            return true;
        } else {
            return false;
        }
    }

    function checkUserName($userName) {
        $obj = json_decode($this->getResponse("http://stopforumspam.com/api?username=" . $userName . "&f=json"));
        if (isset($obj->username->frequency) && $obj->username->frequency != 0) {
            return true;
        } else {
            return false;
        }
    }

    function checkHostName($host_name) {
        return filter_var(gethostbyname($host_name), FILTER_VALIDATE_IP);
    }

    function getResponse($request) {
        $curl = curl_init($request);
        curl_setopt($curl, CURLOPT_HEADER, 0);
        curl_setopt($curl, CURLOPT_POST, 0);
        curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, 0);
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
        //curl_setopt ($curl, CURLOPT_SSL_VERIFYHOST, 1);
        $response = curl_exec($curl);
        curl_close($curl);
        return $response;
    }
}