Google Calendar API connection with private key file

bootstrap.php

require_once 'vendor/google-api-php-client/src/Google/autoload.php';

// email address from the client
$serviceAccountName = '####################@developer.gserviceaccount.com';

$scopes = array(
	'https://www.googleapis.com/auth/calendar',
	'https://www.googleapis.com/auth/calendar.readonly'
);

$privateKey = file_get_contents(__DIR__.'/private-key.p12');

$credentials = new Google_Auth_AssertionCredentials($serviceAccountName, $scopes, $privateKey);

$client = new Google_Client();
$client->setApplicationName('My Application');
$client->setAssertionCredentials($credentials);

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

$service = new Google_Service_Calendar($client);

events.php

/*
 * Example of how to get list of events for each calendars shared through the api
 */

$calendars = $service->calendarList->listCalendarList();

foreach ($calendars->getItems() as $calendar) {
	echo '<p><strong>' . $calendar->getSummary() . '</strong> ' . $calendar->getId() . '</p>';

	$params = array(
		'singleEvents' => true,
		'orderBy' => 'startTime',
		'timeMin' => (new DateTime())->format(DateTime::RFC3339),
		'timeMax' => (new DateTime())->add(new DateInterval('P1D'))->format(DateTime::RFC3339),
	);

	$events = $service->events->listEvents( $calendar->getId(), $params );

	foreach ($events->getItems() as $event) {
		echo '<p>';
		echo "Summary:  ", $event->getSummary(), "<br />";
		echo "Location: ", $event->getLocation(), "<br />";
		echo "Start:    ", $event->getStart()['date'], "<br />";
		echo "End:      ", $event->getEnd()['date'], "<br />";
		echo '</p>';
	}
}