This sample code uses Google Service Accounts with PHP to get a list of users in a Google Apps Domain using the Admin Directory API.

To get started, install the Google API PHP library, create a Google Service account with domain-wide delegation and save the private JSON file in the same directory as your PHP application.

<?

  require_once __DIR__ . '/vendor/autoload.php';

  define('APPLICATION_NAME', 'Service Accounts Tutorial');
  define('SCOPES', implode(' ', array(
    Google_Service_Drive::DRIVE,
    Google_Service_Gmail::MAIL_GOOGLE_COM,
    Google_Service_Directory::ADMIN_DIRECTORY_USER_READONLY,
    Google_Service_People::USERINFO_EMAIL
  )));

  putenv('GOOGLE_APPLICATION_CREDENTIALS=private_key.json');
  date_default_timezone_set("Asia/India");

  $user = "amit@labnol.org";

  $client = new Google_Client();
  $client->useApplicationDefaultCredentials();
  $client->setApplicationName(APPLICATION_NAME);
  $client->setScopes(SCOPES);
  $client->setAccessType("offline");
  $client->setSubject($user);

  $service = new Google_Service_Directory($client);

  $optParams = array(
    'domain' => 'ctrlq.org',
    'maxResults' => 10,
    'orderBy' => 'email'
  );

  $results = $service->users->listUsers($optParams);

  print_r($results);

?>