/**
  * @param ClientInterface $client
  * @param GiropayTransactionStartRequest $giropayRequest
  * @return RequestInterface
  */
 protected function buildTransactionStartRequest(ClientInterface $client, GiropayTransactionStartRequest $giropayRequest)
 {
     $requestArray = array();
     $requestArray['merchantId'] = $this->getMerchantId();
     $requestArray['projectId'] = $this->getProjectId();
     if ($giropayRequest->getMerchantTxId() == "") {
         throw new InvalidArgumentException("Field merchantTxId is required");
     }
     $requestArray['merchantTxId'] = $giropayRequest->getMerchantTxId();
     if ($giropayRequest->getAmount() == "") {
         throw new InvalidArgumentException("Field amount is required");
     }
     $requestArray['amount'] = $giropayRequest->getAmount();
     if ($giropayRequest->getCurrency() == "") {
         throw new InvalidArgumentException("Field currency is required");
     }
     $requestArray['currency'] = $giropayRequest->getCurrency();
     if ($giropayRequest->getPurpose() == "") {
         throw new InvalidArgumentException("Field purpose is required");
     }
     $requestArray['purpose'] = $giropayRequest->getPurpose();
     if ($giropayRequest->getBic() == "") {
         throw new InvalidArgumentException("Field bic is required");
     }
     $requestArray['bic'] = $giropayRequest->getBic();
     if ($giropayRequest->getIban() != "") {
         $requestArray['iban'] = $giropayRequest->getIban();
     }
     if ($giropayRequest->getInfo1Label() != "" || $giropayRequest->getInfo1Text() != "") {
         $requestArray['info1Label'] = $giropayRequest->getInfo1Label();
         $requestArray['info1Text'] = $giropayRequest->getInfo1Text();
     }
     if ($giropayRequest->getInfo2Label() != "" || $giropayRequest->getInfo2Text() != "") {
         $requestArray['info2Label'] = $giropayRequest->getInfo2Label();
         $requestArray['info2Text'] = $giropayRequest->getInfo2Text();
     }
     if ($giropayRequest->getInfo3Label() != "" || $giropayRequest->getInfo3Text() != "") {
         $requestArray['info3Label'] = $giropayRequest->getInfo3Label();
         $requestArray['info3Text'] = $giropayRequest->getInfo3Text();
     }
     if ($giropayRequest->getInfo4Label() != "" || $giropayRequest->getInfo4Text() != "") {
         $requestArray['info4Label'] = $giropayRequest->getInfo4Label();
         $requestArray['info4Text'] = $giropayRequest->getInfo4Text();
     }
     if ($giropayRequest->getInfo5Label() != "" || $giropayRequest->getInfo5Text() != "") {
         $requestArray['info5Label'] = $giropayRequest->getInfo5Label();
         $requestArray['info5Text'] = $giropayRequest->getInfo5Text();
     }
     $requestArray['urlRedirect'] = $giropayRequest->getUrlRedirect();
     $requestArray['urlNotify'] = $giropayRequest->getUrlNotify();
     //this works because the hash list in PHP has a sort order (we get the values out in the order we added them)
     $sortedValuesString = implode('', array_values($requestArray));
     $requestArray['hash'] = $this->getHMACMD5Hash($this->getSecret(), $sortedValuesString);
     $request = $client->createRequest("POST", "https://payment.girosolution.de/girocheckout/api/v2/transaction/start", ['body' => $requestArray]);
     return $request;
 }