With my link checking tool I just created, people can use it for free, but they have a limited number of links that they can track. Paid users have more options, and can track more links. So, I needed to be able to use PayPal subscriptions, and I didn't want to have to manually initiate and track them. PayPal publishes their APIs, but I also didn't want to spend days building a PHP class to interface with those APIs! So I did a quick Google search, and was able to find an open source PayPal IPN class. The class was perfect, it allowed me to create and handle subscription info passed from PayPal. Now I can track when users sign up, when the pay, and even when they cancel!