$session = new Session();
$myStorage = new DatabaseConnection(DB_HOST, DB_USER, DB_PASS, DB_NAME);
// sent by OAuth, you should know that
$state = $_GET['state'];
// personal state
$code = $_GET['code'];
// follow the OAuth protocol and get an access token and a refresh token
if ($client->authorizationCodeCaught()) {
    // make the next OAuth call to get those tokens (crendentials)
    $response = $client->getCredentials();
    if ($response->getCode() !== 200) {
        throw new \Exception(sprintf("An OAuth error occured: %s", $response->getErrorMessage()));
    }
    // now merge or create our user if it does not exist !
    // $appUser     = $myStorage->getUser($session->getUserId());
    $mediUser = $client->getUser();
    $appUser = $myStorage->getUserByUsername($mediUser->email);
    // i chose to merge by email/username
    if ($appUser) {
        // my user alreay exists !
        // I just have to connect him to my app and update his tokens if they are expired
        $myStorage->updateUserCredentials($appUser->id, array('access_token' => $client->getAccessToken(), 'refresh_token' => $client->getRefreshToken(), 'expires_in' => $client->getTokenLifeDuration()));
        $session->setUserId($appUser->id);
        header('Location: index.php');
    } else {
        // user dont exist on my app, lets create it
        $userId = $myStorage->createUser(array('username' => $mediUser->email, 'password' => 'test', 'access_token' => $client->getAccessToken(), 'refresh_token' => $client->getRefreshToken(), 'expires_in' => $client->getTokenLifeDuration()));
        $session->setUserId($userId);
        header('Location: index.php');
    }
}