How do I receive and record Paypal transaction variables from POST? I have a service I want my customers to subscribe to, and I want to be able to update a field within my DB based on their subscription. Was it cancelled? Did it successfully begin/recur?  I'd like to get this information, so I can perform the update within my database accordingly.

I would like to capture the transaction variables that Paypal POSTS to a specified successful payment URL, but I have no idea how. I obviously need to create a page that does this, but I'd like to know what I need to do.
Look at the uc_paypal module included with Ubercart for a fantastic example of this. Look at uc_paypal_menu() (in uc_paypal.module) and uc_paypal_ipn() (in uc_paypal.pages.inc).