Pyrocms Google URL shortener and the oAuth2 conundrum

pyro_outh2.php

<?php
defined('BASEPATH') or exit('No direct script access allowed');

class Plugin_Googleshort extends Plugin
{
    const CLIENT_ID = 'xxxxx.apps.googleusercontent.com';
    const SERVICE_ACCOUNT_NAME = '[email protected]';
    const KEY_FILE = 'xxxxx-privatekey.p12';
    
    function shorten()
    {
        require_once 'googleAPI/src/Google_Client.php';
        require_once 'googleAPI/src/contrib/Google_UrlshortenerService.php';
               
        $client = new Google_Client();
        $client->setApplicationName("Short URL");
        
        // Set your cached access token. Remember to replace $_SESSION with a
        // real database or memcache
        session_start();  // of course, I can't do this because session headers have already been set
        if (isset($_SESSION['token'])) {
            $client->setAccessToken($_SESSION['token']);
        }
        
        // Load the key in PKCS 12 format (you need to download this from the
        // Google API Console when the service account was created.

        $key = file_get_contents(Plugin_Googleshort::KEY_FILE);
        $client->setAssertionCredentials(new Google_AssertionCredentials(Plugin_Googleshort::SERVICE_ACCOUNT_NAME, array(
            'https://www.googleapis.com/auth/urlshortener'
        ), $key, 'notasecret'));
        
        $client->setClientId(Plugin_Googleshort::CLIENT_ID)
        
        $accessToken = $client->getAccessToken(); // nice idea, but this doesn't work of course.
        $this->load->driver('Streams');
        $entry_data = array('google_session' => $accessToken);

        $this->streams->entries->update_entry('1', $entry_data, 'google_sessions', 'streams');
               
        $service = new Google_UrlshortenerService($client);
        
        $url          = new Google_Url();
        $url->longUrl = "http://www.bbc.co.uk/";
        $short        = $service->url->insert($url);
        var_dump($short);
        
        // We're not done yet. Remember to update the cached access token.
        // Remember to replace $_SESSION with a real database or memcached.
        //if ($client->getAccessToken()) {
        //  $_SESSION['token'] = $client->getAccessToken();
        //}
    }
}