Using a Service Account, getAccessToken() is returning null

Question

I am running the following PHP code, using the client libraries found here: https://code.google.com/p/google-api-php-client/. I do not get any errors for any of this code,

Answer 1

For some reason, this seemed to work:

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

const CLIENT_ID = 'blahblahblah';
const SERVICE_ACCOUNT_NAME = 'blahblahblah@developer.gserviceaccount.com';
const KEY_FILE = 'path/to/privatekey.p12';
const CALENDAR_SCOPE = "https://www.googleapis.com/auth/calendar";

$key = file_get_contents(KEY_FILE);
$auth = new Google_AssertionCredentials(
    SERVICE_ACCOUNT_NAME,
    array(CALENDAR_SCOPE),
    $key
);

$client = new Google_Client();
$client->setScopes(array(CALENDAR_SCOPE));
$client->setAssertionCredentials($auth);
$client->getAuth()->refreshTokenWithAssertion();
$accessToken = $client->getAccessToken();

$client->setClientId(CLIENT_ID);

If someone can explain why this worked, please edit this answer or comment!