I'm developing an application in PHP that involves making monthly subscriptions via Paypal.
As this is a subscription service, I have activated the Instant Payment Notification (IPN) in the Paypal panel.
I'm already getting the IPNs normally, and I can perform certain actions according to the content I receive.
My problem is how to integrate between my page and Paypal. I have not yet figured out a way to create a signature button that allows me to send a custom field from my tool.
For example: I have a field in my database that allows me to identify the user who is about to perform the signature. How do I proceed to send this field to Paypal so that I receive this value in the IPNs?