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.