Unless you're using a recent version of PHP, in which case they're all in the $GET array under their own names; e.g. $GET['amount], $_GET['response_code'], etc.
In either case, they should also be accessable through the $HTTP_GET_VARS array in the same was as through $_GET; it's advisable to use one of these two arrays for safety purposes - that way you can guarantee that the variables you're using are getting their values from where they're supposed to be.
But was that URL written correctly? 'Cos if it wasn't, what I said would be useless.