/**
  * Perform renew operation
  * 
  * @param str $domainName
  * @param int $period Renew period in years
  * @param 
  * 
  * @return str json response
  */
 public function renew($domainName, $period, $currentExpirationDate, $cltrid = false)
 {
     $data = array('period' => array('unit' => 'y', 'value' => (int) $period), 'curExpDate' => CommonFunctions::dateFormat($currentExpirationDate, 'c'));
     $json = APIRequest::POST(sprintf("/domains/%s/renew", $domainName), $cltrid ?: ApiRequest::defaultClientTransactionID(), STRegistry::Session()->getAuthToken(), array(), $data);
     return $json;
 }