On the Zend Developer Zone there's a recent article about using APIs, specifically on how to use the Digg API with the Services_Digg2 PEAR package.

A few weeks ago, a client asked me to add a feed of interesting news stories to his Web application. Naturally, my thoughts turned immediately to Digg, which invariably has something interesting to read and which also offers a Web service API [...] A little Googling, and I found the PEAR Services_Digg2 class, which exposes a neat little PHP interface to the Digg API. As you might imagine, with all these tools to hand, it didn't take long to quickly integrate a feed of Digg stories into the application.

He walks you through the installation of the package (a one command step) and a secondary package you'll need due to Digg's authentication, HTTP_OAuth. He includes a request and response example (returned in JSON) as well as several code examples for sample requests, searching, working with comments on posts, post comments, "digg" stories and follow other users.