I use payflow link and when my customers return to my site all of my variables are returned to me as hidden fields to the URL I choose. You probably don't want the variables to transfer in the URL because they can get lost. Most credit card processors should have a way to get the variables passed more efficiently than URL passing.
Once you have them you can update a database, put them into a text file, or just email yourself the variables using the mail function. If you want to populate a form on your site use php print or echo.
<?php print stripslashes($variable) ?>
Put that in your value="" part of the form and they will print right out.
Hope this helps.