HybridAuth, open source social sign on php library

HybridAuth enable developers to easily build social applications to engage websites vistors and customers on a social level by implementing social signin, social sharing, users profiles, friends list, activities stream, status updates and more.

HybridAuth goal is to act as an abstract api between your application and various social apis and identities providers such as Facebook, Twitter, MySpace and Google.

Source: http://hybridauth.sourceforge.net/index.html