Using Paypal IPN, you can verify sales and order to your digital products like ebook or software. You can use PHP to create a simple script that will communicate with Paypal IPN to do that. And depending on the result of the "communication", you can later serve or deny access to your product. This simple tutorial will show you how Paypal IPN works and demonstrate a simple Paypal IPN handler that you can use. You'll be using PHP to create your IPN handler.