I've been looking into OAuth recently and really like what I see, so I started looking at actually starting to play with something that uses it (and isn't twitter). In the pursuit of this, I spent some time walking through the process of how to actually authenticate using OAuth, as a client. I chose Yahoo!'s service, because they have some fabulous developer documentation and have a standard OAuth implementation. Although you don't strictly need any special libraries to handle OAuth, that would be a bit like decoding XML with a regex, so I used the OAuth Package from PECL. For others (including me after I've slept), here's an outline of the process.