Freek Lijten has posted a great introduction to using OAuth in PHP via the PECL OAuth package.

If you're a webdeveloper and haven't heard of OAuth yet, you've been living under a rock. Apparently you moved though as you're reading this. In this article I'll try to explain the concept behind OAuth, give a quick overview of the technique behind it and give a real-world example using the Twitter API and a PECL package called OAuth.

He introduces you to some of the concepts behind OAuth, what it stands for and what problem it solves, and includes a graphic showing how a typical OAuth request happens. Next up is some code, a sample connection to an OAuth-based service to fetch a request token and apply it to your requests. His example makes a request to the Twitter API.