On the SitePoint PHP blog there's a new tutorial today showing you how to setup a user login through PayPal that lets users authenticate for your application through PayPal's systems.

Curiosity is one of the most important traits in our job. The other day, I found myself exploring PayPal documentation to find something interesting to learn (and share). After a while I stumbled upon the Log In with PayPal tool. With the “Log In with PayPal” tool, your users can authenticate into your application using PayPal. It’s the same procedure we already know for Facebook, or maybe Twitter and GitHub. Using this type of authentication is recommended if you want to integrate it with an e-commerce website, but you can use it in every situation and application that requires a user account or membership.

He starts by answering the "why use it" question, suggesting that it adheres to one of the main goals of good, secure authentication systems - simplicity. He then shares an overview of how the process flow works including a graphic outlining each piece involved and what kinds of data is transmitted at each step. He then walks you through the full process of setting up a PayPal application on your account and using the Httpful library (installed via Composer) to connect to their API. He includes the code you'll need to include in your application to provide the link to PayPal for the login and the page it will return to once the process is complete.