index.php

<?php

require_once 'vendor/autoload.php';

$opts = [
    'calendar-id'   => 're2h5k3qcl80edoihv27bn6i3c@group.calendar.google.com',
    'client-email'  => 'xxxxxxxxxxxxxxxxxxxxx',
    'private-key'   => file_get_contents('auth-key.p12'),
    'scopes'        => [
        'https://www.googleapis.com/auth/calendar'
    ],
];

$credentials = new Google_Auth_AssertionCredentials(
    $opts['client-email'], 
    $opts['scopes'], 
    $opts['private-key']
);

$client = new Google_Client();
$client->setAssertionCredentials($credentials);

if ($client->getAuth()->isAccessTokenExpired()) {
  $client->getAuth()->refreshTokenWithAssertion();
}

$service = new Google_Service_Calendar($client);


$event = new Google_Service_Calendar_Event();
$event->setSummary('Appointment');
$event->setLocation('Somewhere');
$start = new Google_Service_Calendar_EventDateTime();
$start->setDateTime('2015-05-20T10:00:00.000-07:00');
$event->setStart($start);
$end = new Google_Service_Calendar_EventDateTime();
$end->setDateTime('2015-05-25T10:25:00.000-07:00');
$event->setEnd($end);
$attendee1 = new Google_Service_Calendar_EventAttendee();
$attendee1->setEmail('email@domain.com');

$attendees = array($attendee1);
$event->attendees = $attendees;
$createdEvent = $service->events->insert($opts['calendar-id'], $event);

echo $createdEvent->getId();