Use Google Calendar API

addAttendee.php

<?php
include_once 'google-api-php-client/vendor/autoload.php';

$client = new Google_Client();

$application_creds = 'service-account-credentials.json';

$credentials_file = file_exists($application_creds) ? $application_creds : false;
define("SCOPE",Google_Service_Calendar::CALENDAR);
define("APP_NAME","Google Calendar API PHP");

$client->setAuthConfig($credentials_file);
$client->setApplicationName(APP_NAME);
$client->setScopes([SCOPE]);

$service = new Google_Service_Calendar($client);

$id = 'event-id';
$event = $service->events->get('primary',$id);

$attendeeNew = new Google_Service_Calendar_EventAttendee();
$attendeeNew->setEmail('[email protected]');
$attendeeNew->setResponseStatus('accepted');
$attendeeNew->setOrganizer(true);

$attedess = $event->getAttendees();
array_push($attedess,$attendeeNew);
$event->setAttendees($attedess);

$updatedEvent = $service->events->update('primary', $id, $event);

var_dump($updatedEvent->getAttendees());

createEventArray.php

<?php
include_once 'google-api-php-client/vendor/autoload.php';

$client = new Google_Client();

$application_creds = 'service-account-credentials.json';

$credentials_file = file_exists($application_creds) ? $application_creds : false;
define("SCOPE",Google_Service_Calendar::CALENDAR);
define("APP_NAME","Google Calendar API PHP");

$client->setAuthConfig($credentials_file);
$client->setApplicationName(APP_NAME);
$client->setScopes([SCOPE]);

$service = new Google_Service_Calendar($client);

$event = new Google_Service_Calendar_Event(array(
	'summary' => 'Google I/O 2015',
  	'location' => '800 Howard St., San Francisco, CA 94103',
  	'description' => 'A chance to hear more about Google\'s developer products.',
  	'start' => array(
    	'dateTime' => '2015-12-01T10:00:00.000-05:00',
    	'timeZone' => 'America/Los_Angeles',
  	),
  	'end' => array(
    	'dateTime' => '2015-12-01T10:00:00.000-05:00',
    	'timeZone' => 'America/Los_Angeles',
  	),
  	'attendees' => array(
    	array(
    		'email' => '[email protected]',
    		'organizer' => true
    		),
    	array(
    		'email' => '[email protected]',
    		'resource' => true
		),
  	),
  	"creator"=> array(
    	"email" => "[email protected]",
    	"displayName" => "Example",
    	"self"=> true
  	),
  	"guestsCanInviteOthers" => false,
  	"guestsCanModify" => false,
  	"guestsCanSeeOtherGuests" => false,
));

$calendarId = 'primary';
$event = $service->events->insert($calendarId, $event);
printf('Event created: %s', $event->htmlLink);

google-calendar.php

<?php
include_once 'google-api-php-client/vendor/autoload.php';

$client = new Google_Client();

$application_creds = 'service-account-credentials.json';

$credentials_file = file_exists($application_creds) ? $application_creds : false;
define("SCOPE",Google_Service_Calendar::CALENDAR);
define("APP_NAME","Google Calendar API PHP");

$client->setAuthConfig($credentials_file);
$client->setApplicationName(APP_NAME);
$client->setScopes([SCOPE]);

$service = new Google_Service_Calendar($client);

$event = new Google_Service_Calendar_Event();  
$event->setSummary('Halloween');
$event->setLocation('The Neighbourhood');
$event->setSummary('Evento creado desde el API')

$start = new Google_Service_Calendar_EventDateTime();
$start->setDateTime('2015-12-01T10:00:00.000-05:00');
$event->setStart($start);

$end = new Google_Service_Calendar_EventDateTime();
$end->setDateTime('2015-12-02T10:25:00.000-05:00');
$event->setEnd($end);

$attendee = new Google_Service_Calendar_EventAttendee();
$attendee->setEmail('[email protected]');
$attendees = array($attendee);
$event->setAttendees($attendees);

$organizer = new Google_Service_Calendar_EventOrganizer();
$organizer->setEmail('[email protected]');
$organizer->getDisplayName("Full Name");

$event->setOrganizer($organizer);

$createdEvent = $service->events->insert('primary', $event);
echo "ID => <br>"; 
echo $createdEvent->getId() . "<br>";
echo "HTML => <br>";
echo $createdEvent->getHtmlLink() . "<br>";

?>