Trying to run a PHP script after PayPal payment
 

Ok, so I'm trying to setup some way of running a custom PHP script after a user has finished paying their subscription though the Paypal gateway. I need to be able to pass the subscription usergroup they were put in according to what they signed up for to this script. The script will then insert a user to a custom database based on what usergroup they were put into. I've looked at various threads, but haven't found anything that worked. Any help would be appreciated.