This library now implements draft 20 of OAuth 2.0. The client is still only draft-10.
This version of oauth2-php is a fork of https://github.com/quizlet/oauth2-php with the following changes:
(pull request is pending)
https://github.com/quizlet/oauth2-php is a fork of http://code.google.com/p/oauth2-php/ updated against OAuth2.0 draft 20, with a better OO design.
http://code.google.com/p/oauth2-php/ is the original repository, which seems abandonned.