Google Service Account Authentication PHP

I am trying to authenticate a service account so that I can use the access token with the client JSON_API library.

I have viewed these articles:

https://code.google.com/p/google-api-php-client/source/browse/trunk/examples/prediction/serviceAccount.php
https://code.google.com/p/google-api-php-client/wiki/UsingTheLibrary

https://developers.google.com/storage/docs/authentication#service_accounts https://developers.google.com/accounts/docs/OAuth2#scenarios

Here's my PHP Code

<?php

require_once 'google-api-php-client/src/Google_Client.php';

const CLIENT_ID = "";
const SERVICE_ACCOUNT_NAME = "";
const KEY_FILE = "super secret path of course ;)";

$client = new Google_Client();

// Loads the key into PKCS 12 format
$key = file_get_contents(KEY_FILE);
$client->setAssertionCredentials(new Google_AssertionCredentials(
    SERVICE_ACCOUNT_NAME,
    array('https://www.googleapis.com/auth/prediction'),
    $key
  )
);

$client->setClientId(CLIENT_ID);
$auth = $client->authenticate();
print $auth ? "Returned true" : "Returned false";
print "<br>";
print is_null($client->getAccessToken()) ? "It's null" : "Works";

?>

Here's my output:

Returned true
It's null

I finally figured out how to authenticate using the PHP API library after using a mixture of different resources.

Here's my authentication class for the google php api libray

<?php
require_once 'google-api-php-client/src/Google_Client.php';
require_once 'google-api-php-client/src/contrib/Google_StorageService.php';

class Model_Storage_Auth
{
    const CLIENT_ID = "someuniquenumber.apps.googleusercontent.com";
    const SERVICE_ACCOUNT_NAME = "myserviceaccountname@developer.gserviceaccount.com";
    const KEY_FILE = "/supersecretpath/key.p12";
    const ACCESS_TOKEN = 'access_token';
    const APP_NAME = 'My App Name';

    private $google_client;

    function __construct()
    {
        $this->google_client = new Google_Client();
        $this->google_client->setApplicationName(self::APP_NAME);
    }

    public function getToken()
    {
        if(!is_null($this->google_client->getAccessToken())){}
        elseif(!is_null(Session::get(self::ACCESS_TOKEN, null)))
        {
            $this->google_client->setAccessToken(Session::get(self::ACCESS_TOKEN, null));
        }
        else
        {
            $scope = array();
            $scope[] = 'https://www.googleapis.com/auth/devstorage.full_control';
            $key = file_get_contents(self::KEY_FILE);
            $this->google_client->setAssertionCredentials(new Google_AssertionCredentials(
                self::SERVICE_ACCOUNT_NAME,
                $scope,
                $key)
            );
            $this->google_client->setClientId(self::CLIENT_ID);
            Google_Client::$auth->refreshTokenWithAssertion();
            $token = $this->google_client->getAccessToken();
            Session::set(self::ACCESS_TOKEN, $token);
        }
        return $this->google_client->getAccessToken();
    }

}