PHPMaster.com: Creating a PHP OAuth Server
If you've ever integrated with another API that requires security (such as Twitter), you've probably consumed an OAuth service. In this article, I'll explore what it takes to create your own three-legged OAuth server allowing you, for example, to create your own secure API which you can release publicly.
They include a visual representation of the OAuth authentication flow (it's not the simplest thing) and the database structure/sample code you'll need to get the server up and listening. Also included is a registration form and how to generate a request token and give back an access token. There's also some sample code showing how to validate the request and it's access token to check for a correct (and allowed) request.