I use this as a quick and crude workaround to use a Zimbra calendar with a self-signed SSL certificate and HTTP authentication in Google Calendar:
Code:
<?php
header('Content-Type: text/calendar; charset=utf-8');
header('Content-Disposition: attachment; filename="calendar.ics"; ');
print file_get_contents("https://myuser:mypass@myserver/zimbra/home/myuser/calendar.ics");
?>