The PayPal application programming interface (API) has been around from about 2006, and is a more secure way of performing a number of different actions without having to login to PayPal itself. So you can initiate, improve and automate PayPal functions to more suit your applicational needs, as opposed to say, screen scraping. To follow this tutorial, you'll need a reasonable grasp of PHP, a Business PayPal account (if you have a Premier account, just upgrade: for free!), and some patience to make it work.
Coding Library: PHP