google-example.php

    <?php
    /**
     * 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';

    /**
     * Client ID from https://console.developers.google.com/
     * Must be added in Google Apps Admin console under Security -> Advanced -> Manage API client access
     * Requires scope https://www.googleapis.com/auth/admin.directory.user or
     * https://www.googleapis.com/auth/admin.directory.user.readonly
     */
    $clientId = 'somelongstring.apps.googleusercontent.com';

    /**
     * Service Account Name or "Email Address" as reported on https://console.developers.google.com/
     */
    $serviceAccountName = 'somelongstring@developer.gserviceaccount.com';

    /**
     * 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 = 'delegated-admin@domain.com';

    /**
     * This is the .p12 file the Google Developer Console gave you for your app
     */
    $keyFile = 'file.p12';

    /**
     * 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'
    );

    /**
     * Create AssertionCredentails object for use with Google_Client
     */
    $creds = new Google_Auth_AssertionCredentials(
        $serviceAccountName,
        $scopes,
        file_get_contents($keyFile)
    );
    /**
     * This piece is critical, API requests must be used with sub account identifying the
     * delegated admin that these requests are to be processed as
     */
    $creds->sub = $delegatedAdmin;

    /**
     * Create Google_Client for making API calls with
     */
    $client = new Google_Client();
    $client->setApplicationName($appName);
    $client->setClientId($clientId);
    $client->setAssertionCredentials($creds);

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

    /**
     * Get specific user example
     */
    //$account = $dir->users->get('example@domain.com');
    //print_r($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 = $dir->users->listUsers(array('domain' => 'domain.com', 'maxResults' => 100));
    print_r($list);

other ref:
http://michaelseiler.net/2014/12/16/google-admin-sdk-api-with-php/
http://stackoverflow.com/questions/26038188/google-admin-sdk-php-cannot-create-user-not-authorized