How to insert event into a private Google Calendar with PHP? - php

Question

I'm trying to add new events into my private Google Calendar, but I get (403) Forbidden message. I've searched the internet but I found no answer to my issue.
My scope is set to read/write, and the credentials should be ok, because I can read out all of my events so the connection works fine.
Here's the code:
$service_account = $this->service_account;
$json_key = json_decode(file_get_contents($this->json_url));
$scopes = 'https://www.googleapis.com/auth/calendar';
$application_name = 'Calendar';
$client = new Google_Client();
$client->setApplicationName($application_name);
$cred = new Google_Auth_AssertionCredentials($service_account, array($scopes), $json_key->private_key);
$client->setAssertionCredentials($cred);
if($client->getAuth()->isAccessTokenExpired()) {
$client->getAuth()->refreshTokenWithAssertion($cred);
}
$google_service = new Google_Service_Calendar($client);
$calendar_ids = $this->calendar_ids;
$event = new Google_Service_Calendar_Event();
$event->setSummary($service);
$event->setDescription($user);
$event->setLocation($location);
$start = new Google_Service_Calendar_EventDateTime();
$start->setDateTime($this->sqlDateToGcalDate($start_date));
$event->setStart($start);
$end = new Google_Service_Calendar_EventDateTime();
$end->setDateTime($this->sqlDateToGcalDate($end_date));
$event->setEnd($end);
$google_service->events->insert($calendar_ids[$steed], $event);
$google_service->events->insert($calendar_ids[$location], $event);