Zend Framework has pretty good OAuth consumer implementation. However, it has no support for implementing OAuth provider, and it turns out that there aren't many other libraries for it. Most examples out there base on PECL oauth extension, which works just fine, with one caveat - you have to have this PECL extension installed, while ZF implementation does not require that. So I went ahead and wrote some code that allows to easily add OAuth provider to your ZF-based or ZF-using application. That should make writing OAuth provider easier.
His code just fleshes out the server portion of the provider, not all of the token generation and key handling it'll need on the backend - that'll still be the job of your scripts. You can find the library over on github in his Zend_OAuth_Provider repository.