On PHPMaster.com there's a new tutorial showing you how to implement OAuth authentication in your application using the Opauth Php library. This library lets you connect to any of a number of OAuth service providers and authenticate.

Open Authentication has evolved as a standard for third-party authentication in recent years and allows us to securely authenticate our application's users through a standard interface. Twitter and Facebook has been the standouts among dozens of authentication service providers. [...] In this article we'll explore how we can effectively use Opauth to standardize our authentication strategies. I'll be using CodeIgniter here, but even if you're not familiar with CodeIgniter, I suggest you continue reading since it will be similar for other frameworks as well. Once you understand the necessary details for integration, adapting to any framework is super simple.

He talks some about the Opauth library and shows the full authentication flow that your script will follow when using it. Code is included showing how to include and integrate the Opauth library including setup and configuration of the object and the controller/view code to implement the login form.