Are you sure the form is being posted to register.php in the first place, and not payment.php?  Note that the URL that delivers the form (what you see in the browser's location bar) is not necessarily the one the form is submitted to (rather what's specified in the action attribute of the form).

Otherwise, there might also be a redirect being done.  To find out what exactly is going on, trace the HTTP using Firefox's Live HTTP Headers addon, or a similar tool.