Google_Service_Directory - (403) Not Authorized to access this resource/api

I have a problem just using an example of actual version of PHP api, and using the "service-account.php" file of examples folder.

the original is for show the "Books API", and with my personal credentials configuration it works well, but in my xcase I need to access by directory.groups.get service to have the list of members accounts of a google groups mail list, so I change the original code in this:

<?php

session_start();
include_once "templates/base.php";

/************************************************
  Make an API request authenticated with a service
  account.
 ************************************************/
require_once realpath(dirname(__FILE__) . '/../autoload.php');

/************************************************
 ************************************************/

// MY ACCOUNT DATA HERE
$client_id = 'xxx';
$service_account_name = 'xxx'; //Email Address 
$key_file_location = 'xxx.p12'; //key.p12
$groupKey = 'xxx';

echo pageHeader("My Service Account Access");
if ($client_id == '<YOUR_CLIENT_ID>'
    || !strlen($service_account_name)
    || !strlen($key_file_location)) {
  echo missingServiceAccountDetailsWarning();
}

$client = new Google_Client();
$client->setApplicationName("Client_Library_Examples");
//$service = new Google_Service_Books($client); //ORIGINAL
$service = new Google_Service_Directory($client);

/************************************************
 ************************************************/
if (isset($_SESSION['service_token'])) {
  $client->setAccessToken($_SESSION['service_token']);
}
$authArray = array(
                'https://www.googleapis.com/auth/admin.directory.group',
                'https://www.googleapis.com/auth/admin.directory.group.readonly',
                'https://www.googleapis.com/auth/admin.directory.group.member',
                'https://www.googleapis.com/auth/admin.directory.group.member.readonly'
);
$key = file_get_contents($key_file_location);
$cred = new Google_Auth_AssertionCredentials(
    $service_account_name,
    $authArray, //array('https://www.googleapis.com/auth/books'), //ORIGINAL
    $key
);
$client->setAssertionCredentials($cred);
if($client->getAuth()->isAccessTokenExpired()) {
  $client->getAuth()->refreshTokenWithAssertion($cred);
}
$_SESSION['service_token'] = $client->getAccessToken();


/************************************************
 ************************************************/
//$optParams = array('filter' => 'free-ebooks'); //ORIGINAL
$optParams = array('fields' => 'id');
//$results = $service->volumes->listVolumes('Henry David Thoreau', $optParams); //ORIGINAL
$results = $service->groups->get($groupKey, $optParams);
echo "<h3>Results Of Call:</h3>";
foreach ($results as $item) {
  //echo $item['volumeInfo']['title'], "<br /> \n"; //ORIGINAL
    echo "<pre>".print_r ($item, true)."</pre>";
}

echo pageFooter(__FILE__);

whatever I do, providing authorization for API SDK, and using file and credentials just created in API Credentials panel of the console's developer, I receive alwais the 403 error.

Here's the error stack:

#0 /var/www/html/google_local/google-api-php-client-master/src/Google/Http/REST.php(41): 
Google_Http_REST::decodeHttpResponse(Object(Google_Http_Request)) 
#1 /var/www/html/google_local/google-api-php-client-master/src/Google/Client.php(546): 
Google_Http_REST::execute(Object(Google_Client), Object(Google_Http_Request)) 
#2 /var/www/html/google_local/google-api-php-client-master/src/Google/Service/Resource.php(190): 
Google_Client->execute(Object(Google_Http_Request)) 
#3 /var/www/html/google_local/google-api-php-client-master/src/Google/Service/Directory.php(1494): 
Google_Service_Resource->call('get', Array, 'Google_Service_...') 
#4 /var/www/html/google_local/googl in /var/www/html/google_local/google-api-php-client-master/src/Google/Http/REST.php on line 76

Any suggestions?

Thanks, Roberto

The root of the problem is that the service account is not an administrator on the domain, so it cannot access the Admin SDK Directory API. Instead, you need to enable domain-wide delegation for your service account, and then have the service account impersonate a domain admin when it makes the request:

$cred = new Google_Auth_AssertionCredentials(
    $service_account_name,
    $authArray,
    $key
);
$cred->sub = "[email protected]";