I've gotten a subscription module setup and I am unsure of what PayPal sends back to my ipn.php file.
Picture this:
My website -> PayPal Payments(for subscription) -> Payment Success -> Process -> ipn.php
What does ipn.php receive?
PayPal will send back the transaction data such as transaction ID, amount, fees amount to your IPN listener depending on the transaction type.
Here's a list of all the variable that IPN might be sending to you. https://developer.paypal.com/webapps/developer/docs/classic/ipn/integration-guide/IPNandPDTVariables/