mike182uk/paypal-ipn-listener — pull request #14 by stefanneubig (2014-11-21 16:43:11+00:00)
Title: Add user agent header to curl request

Because of a recent change in PayPals API you will get a 403 error if you don't send a User Agent header.
Source: https://stackoverflow.com/questions/25854207/access-denied-on-paypal-ipn-verification