Simple OAuth workflow

simple-oauth.php

<?php

$client_id = GOOGLE_CLIENT_ID;
$client_secret = GOOGLE_CLIENT_SECRET;
$redirect_uri = GOOGLE_CALLBACK;

$google_oauth = new Google_OAuth($client_id,$client_secret,$redirect_uri,'userinfo');
$status = '';
$credentials = FALSE;
$errors = array();

/**
 * First attemp, validate any code from Google
 */
if (isset($_REQUEST['code'])) {
    $credentials = $google_oauth->consume_code($_REQUEST['code']);
    $errors []= $credentials;

    if (isset($credentials->email)) {
        /**
         * Do something
         */
    }
    else {
        /**
         * Could not validate the user
         */
        $status = 'failed';
    }
}
else if (isset($_REQUEST['error'])) {
    /**
     * User has denied acces
     */
    $status = 'denied';
}
else {
    /**
     * Redirect to Google Authorization page
     */
    $google_oauth->request_access_code();
}