HybridAuth is an open source library that enables developers to develop social sign-on applications and websites. It provides a set of APIs that allow websites and applications to authenticate users through major social networks such as Facebook, Twitter, Google, Yahoo, LinkedIn and many others. It is based on the OAuth and OpenID protocols, which enable secure user authentication and authorization for third-party applications. It is written in PHP and provides a simple and easy-to-use interface that makes it easy for developers to integrate social networks into their applications. The library also provides support for multiple languages, so that developers can create applications that are accessible to users from different countries.
Discontinued from http://stormpath.com/"Stormpath has joined forces with Okta. The Stormpath API will be shut down on 8/17/2017 at noon PST." Joined http://alternativeapp.info/software/okta/