Query Drive with a service account

drive.php

<?php

require_once "../vendor/autoload.php";

$client = new Google_Client();
$client->setApplicationName("Service Acccount Test");
$client_id = '985145966381-is5nq94smihmoa2k0567jab6u6vj278c.apps.googleusercontent.com';
$service_account_name = '985145966381-is5nq94smihmoa2k0567jab6u6vj278c@developer.gserviceaccount.com';
$key_file_location = 'foostarter-a27f84b2dec4.p12';

$key = file_get_contents($key_file_location);
$cred = new Google_Auth_AssertionCredentials(
    $service_account_name,
    array(Google_Service_Drive::DRIVE_READONLY),
    $key
);
$cred->sub = "ian@example.com"; // For the @example.com Apps delegation.

// Remember to cache the access token locally if making multiple calls
// and don't just use this function for each request!
$client->getAuth()->refreshTokenWithAssertion($cred);

$service = new Google_Service_Drive($client);
$result = $service->files->listFiles();
var_dump($result);