protected function checkSuccess($response)
    {
        if(!isset($response['ACK'])) {
            $this->_success = false;
            $this->addErrorMessage('ACK key not found in response.');
            return;
        } else if($response['ACK'] == "Success" || $response['ACK'] == "SuccessWithWarning") {
            $this->_success = true;
            return;
        }

        if(isset($response['ERRORS'])) {
            foreach($response['ERRORS'] as $error) {
                if(isset($error['LONGMESSAGE'])) {
                    $this->addErrorMessage($error['LONGMESSAGE']);
                }
            }
        } else {
            $this->addErrorMessage('Unknown error has occurred.');
        }

        $this->_success = false;
    }