I hope someone could help with this:
I am working on a php project and I need to implement the authentication of the application as a Service. I need to do something like Google where offers a Service to login in other applications using Google's Authentication Service (for example trello.com allows visitors to login with their google account).
I would like to clarify my mind, because I have no idea how to start it. I am using Symfony2 framework and I'm trying to work RESTFully, but basically what I need is just an idea of what I have to do or investigate to carry this out.