/**
  * Retrieve the CAPTCHA code
  *
  *@param $key
  *
  * @return mixed|null
  */
 protected function getExpectedCode($key)
 {
     $arrayZendSession = new SessionArrayStorage();
     if ($this->session->has($key)) {
         $sessionKey = $this->session->get($key);
         $this->session->remove($key);
         $captchaSession = $arrayZendSession->offsetGet($sessionKey);
         $arrayZendSession->offsetUnset($sessionKey);
         if ($captchaSession instanceof ArrayObject) {
             $word = $captchaSession->offsetGet('word');
             $captchaSession->offsetUnset('word');
             return $word;
         }
     }
     return null;
 }