Google API php Interface

I want to offer people the option of using their Google login & credentials on my site. When they arrive at my site, they would click on a button for a Google login and it takes them to a page where they can choose to allow Google authentication. I actually have this much working on my site. What I cannot figure out is how to have the google API return a unique login value (i.e. user ID) as well as things like their contact information. The script is written in PHP. I am also using the OAuth2 proceedure as outlined by Google on this page: [login to view URL] I need someone to help me get this to the next step where to tell me what is needed to get the Google API to return the above desired items to my php script.