Example of using Google Admin SDK Directory API to get an individual or list of users.

google-example.php

<?php
// Updated 2018-01-24 to work with google/apiclient:^2.0
/*
 * Easiest to use composer to install google-api-php-client and generate autoloader
 * If you dont want to use composer you can manually include any needed files
 */
include_once 'vendor/autoload.php';

/*
 * Email address for admin user that should be used to perform API actions
 * Needs to be created via Google Apps Admin interface and be added to an admin role
 * that has permissions for Admin APIs for Users
 */
$delegatedAdmin = '[email protected]';

/*
 * Some name you want to use for your app to report to Google with calls, I assume
 * it is used in logging or something
 */
$appName = 'Example App';

/*
 * Array of scopes you need for whatever actions you want to perform
 * See https://developers.google.com/admin-sdk/directory/v1/guides/authorizing
 */
$scopes = array(
    'https://www.googleapis.com/auth/admin.directory.user'
);

/*
 * Provide path to JSON credentials file that was downloaded from Google Developer Console
 * for Service Account
 */
$authJson = __DIR__ . '/creds.json';

/*
 * Create Google_Client for making API calls with
 */
$googleClient = new \Google_Client();
$googleClient->setApplicationName($appName);
$googleClient->setAuthConfig($authJson);
$googleClient->setSubject($delegatedAdmin);
$googleClient->setScopes($scopes);
$googleClient->setAccessType('offline');

/*
 * Get an instance of the Directory object for making Directory API related calls
 */
$dir = new \Google_Service_Directory($googleClient);

/*
 * Get specific user example
 */
//$account = $dir->users->get('[email protected]');
//print_r($account);

/*
 * Get list of users example
 * In my testing you must include a domain, even though docs say it is optional
 * I was getting an error 400 without it
 */
$list = $dir->users->listUsers(array('domain' => 'domain.org', 'maxResults' => 500));
print_r($list);