protected function hmac($key = '', $data = '')
    {
        if ($data == '') {
            $this->errorMessage[] = 'DATA FOR HMAC: MISSING!';
            return false;
        }
        if ($key == '') {
            $this->errorMessage[] = 'KEY FOR HMAC: MISSING!';
            return false;
        }
        return hash_hmac('md5', $data, trim($key));
    }