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