title: Google OAuth automatic login
body: <p>I got some question about the OAuth2.0 process.
  I would like to achieve something like Stack <code>Log with google account</code> feature.</p>
  
  <p>I'm using the PHP library from Google.</p>
  
  <p>I'm ok retrieving the refresh token and the access token.</p>
  
  <p>So far so good, my question is : when the user log out from my website and click the button again, how am I supposed to know who the user is to retrieve from DB the right refresh token ?</p>
  
  <p>I thought about keeping the email in a cookie but StackOverflow doesn't seem to do that as I cleared my cookies but Stack keep connecting me as I click the button without showing back the "Application scope authorization" google page.</p>
  
  <p>Am I missing something ? 
  Is there an API allowing to get the user email without token ?
  Maybe RefreshToken should only be used with Mobile APP where you know the user?</p>
  
  <p>What I want to achieve is relatively simple, the user click the sign-in, first time one has to approve the application. Once its done, I would like to automatically connect the user when the sign-in button is clicked.</p>
  
  <p>Thanks for any information on that</p>