How to catch a calendar item created event using push notifications?

Question

I'm trying to fire an event when I create a calendar item (appointment) in exchange, using PHP-EWS. Currently, I am able to fire an event when an email is sent:

    $subscribe_request = new \EWSType_SubscribeType();
    $pushSubscription = new \EWSType_PushSubscriptionRequestType();
    $pushSubscription->StatusFrequency = 1;
    $pushSubscription->URL = 'http://someserver/log.php';
    $folderIDs = new \EWSType_NonEmptyArrayOfBaseFolderIdsType();
    $eventTypes = new \EWSType_NonEmptyArrayOfNotificationEventTypesType();
    $folderIDs->DistinguishedFolderId = new \EWSType_DistinguishedFolderIdType();
    $folderIDs->DistinguishedFolderId->Id = \EWSType_DistinguishedFolderIdNameType::INBOX;

    $eventTypes->EventType = "NewMailEvent";
    $pushSubscription->FolderIds = $folderIDs;
    $pushSubscription->EventTypes = $eventTypes;
    $subscribe_request->PushSubscriptionRequest = $pushSubscription;

    return $this->ews->Subscribe($subscribe_request);

log.php

class ewsService {
  public function SendNotification($arg) {
    file_put_contents("C:\\exchangelogs\\log_".time().".txt", print_r($arg,1));
    $result = new EWSType_SendNotificationResultType();
    $result->SubscriptionStatus = 'OK';
    //$result->SubscriptionStatus = 'Unsubscribe';
    return $result;
  }
}

$opts = array();
$server = new SoapServer(
  'NotificationService.wsdl', 
  array('uri' => 'http://someserver/log.php'));

$server->setObject($service = new ewsService());
$server->handle();

Every minute a 'Keep alive message' is sent and the SendNotification function is called. The same thing happens when a mail is sent (using outlook or whatever).

This all works fine.

However, now I want to do the same when a calendar item is created, such as an appointment. I tried changing the DistinguishedFolderIdNameType to CALENDAR and the EventType to CreatedEvent, but I receive no message when an appointment is created.

Any help is greatly appreciated.