• Created a Project
• Enabled Calendar API
• Created an OAuth2.0 Service Account
• Downloaded the keyfile .p12 and saved it locally (not using a server with a public IP address) Shared my Resource Calendar with the Email address created in the Service Account (with Manage Sharing rights)
• Entered Client ID for the service account and authorized http://www.googleapis.com/auth/calendar

Environment lamp setup on localhost.

<?php

require_once 'google-api-php-client/src/Google/Client.php';
require_once 'google-api-php-client/src/Google/Service/Calendar.php';

session_start();

const CLIENT_ID = 'XXXXXX.apps.googleusercontent.com'; //Service CLIENT ID
const SERVICE_ACCOUNT_NAME = '[email protected]';
const KEY_FILE = 'google-api-php-client/src/Google/Reservation Service-XXXXXXX.p12';

$client = new Google_Client();
$client->setApplicationName("Appointment");


if (isset($_SESSION['token'])) 
{
 $client->setAccessToken($_SESSION['token']);
}

$key = file_get_contents(KEY_FILE);
$client->setClientId(CLIENT_ID);

$client->setAssertionCredentials(new Google_Auth_AssertionCredentials(
  SERVICE_ACCOUNT_NAME,
  array('https://www.googleapis.com/auth/calendar'),
  $key));



//Save token in session
if ($client->getAccessToken())
{
  $_SESSION['token'] = $client->getAccessToken();
}  

$cal = new Google_Service_Calendar($client);

$event = new Google_Service_Calendar_Event();

$event->setSummary('This is a Test event');
$event->setLocation('Test Location');


$start = new Google_Service_Calendar_EventDateTime();
$start->setDateTime('2014-08-20T10:30:00.000-05:00');
$event->setStart($start);

$end = new Google_Service_Calendar_EventDateTime();
$end->setDateTime('2014-08-20T12:30:00.000-05:00');
$event->setEnd($end);

$cal->events->insert('[email protected]', $event);


?>

getting the following error:

Fatal error: Uncaught exception 'Google_Service_Exception' with message 'Error calling POST https://www.googleapis.com/calendar/v3/calendars/XXXXXXX%40resource.calendar.google.com/events: (403) Forbidden' in /google-api-php-client/src/Google/Http/REST.php:79 Stack trace: #0 /google-api-php-client/src/Google/Http/REST.php(44): Google_Http_REST::decodeHttpResponse(Object(Google_Http_Request)) #1 /google-api-php-client/src/Google/Client.php(503): Google_Http_REST::execute(Object(Google_Client), Object(Google_Http_Request)) #2 /google-api-php-client/src/Google/Servic/Resource.php(195): Google_Client->execute(Object(Google_Http_Request)) #3 /google-api-php-client/src/Google/Service/Calendar.php(1459): Google_Service_Resource->call('insert', Array, 'Google_Service_...') #4 /calendar.php(53): Google_S in /google-api-php-client/src/Google/Http/REST.php on line 79

A few people had the same issue, I am sharing the calendar with the service account. Any help will be appreciated.