I cannot seem to find out how to retrieve the transaction reference.
Based on the gateway and the callback url that is called, is this possible?
$purchaseResponse = $gateway->completePurchase()->send();
if($purchaseResponse->isSuccessful())
{
//please give me the transaction id!
(of course I can check it myself using the $_GET parameters, however, that does away with the 'universal' payment provider part.)