If there is more then one implementation, which one is better/mostly maintained?
Specifically for OAuth 2.0 Draft 12.
There is a more up-to-date version of the PHP OAuth2 library available at https://github.com/quizlet/oauth2-php It implements draft 20 of the spec, and will likely work with few or no changes once the final spec is released.
Taken from OAuth Wiki, there seems to be one PHP implementation called oauth2-php which supports IETF draft v9. Since OAuth2 is still a draft, it will probably be hard to find more implementations.
See also the one from Ericsson labs: https://labs.ericsson.com/apis/oauth2-framework/
The OAuth2 Framework allows you to protect your web resources using the next generation OAuth, (http://oauth.net/2/) as well as accessing OAuth2 protected resources, most notably the Facebook Graph API. The API consists of libraries for building your own OAuth2 server as well as client side access. The standard is still in draft mode so expect some level of changes. Currently version 10 of the OAuth 2 specification is the one being supported.