Open source social sign on PHP Library. HybridAuth's 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.
HybridAuth is an open source PHP library to allow you to connect to popular social site APIs (like 
facebook and twitter) and single sign-on services (like openid). It acts as a wrapper for all these services, with a common interface that makes it easier to work with multiple services from a single application.
From their website
HybridAuth enables developers to easily build social applications to engage website visitors and customers on a social level by implementing social signin, social sharing, user profiles, friends list, activities stream, status updates and more.
The goal of HybridAuth is to act as an abstract API between your application and various social APIs and identities providers such as Facebook, Twitter, MySpace and Google.
