PayPal IPN Intro

PayPal will send an IPN message to your app/website when some events occur, your app need to send the original message back to PayPal to verify it.

The code in your app handles this is often called listener/handler, after the IPN message is verified, the listener can trigger other business logic in your app, such as updating order status, handling shipping, sending email etc;

the auth flow works like this:

IPN Auth Flow