All the transaction data is sent, processed, and validated. Then the user is sent on to the web page specified as the next URL. Any page can be configured to follow this hidden payment form.
Click the "Test Hidden Form" button to try it out. View the source code of this page to see how it was done.
The example below is identical to the one above, except that the expiration date has been purposely left out. Click the "Test Hidden Form" example to see how the payment form is displayed when key information is not sent to the server. You can come back here and fill in an expiration date to see it work correctly. Again, view the source code to see how this was done.