Rasmus Lerdorf has written up a guide that can help you create a simple OAuth service with the help of the PECL OAuth extension.

Last year I showed how to use pecl/oauth to write a Twitter OAuth Consumer. But what about writing the other end of that? What if you need to provide OAuth access to an API for your site? How do you do it? Luckily John Jawed and Tjerk have put quite a bit of work into pecl/oauth lately and we now have full provider support in the extension. It's not documented yet at php.net/oauth, but there are some examples in svn.

He walks you through creating the consumer key registration page (so others can signup for access to your provider), making the OAuth endpoints, how to authorize the request token and send back out the access token. He also includes an actual API call to show it working.