title: How to identify a Google OAuth2 user?
body: <p>I used Facebook login to identify users. When a new user comes, I store their userID in my database. Next time they come, I recognized their Facebook ID and I know which user it is in my database.</p>
  
  <p>Now I am trying to do the same with Google's OAuth2, but how can I recognize the users?</p>
  
  <p>Google sends me several codes and tokens (access_token, id_token, refresh_token), however none of them are constant. Meaning if I log out and log back in 2 minutes later, all 3 values have changed. How can I uniquely identify the user?</p>
  
  <p>I am using their PHP client library: <a href="https://code.google.com/p/google-api-php-client/" rel="noreferrer">https://code.google.com/p/google-api-php-client/</a></p>