I have used this php script, create a php file called calsync or something.
header('Content-Type: text/calendar; charset=utf-8');
header('Content-Disposition: attachment; filename="calendar.ics"; ');
Then you can use this url in google calendar.
This can be a big security risk because your eliminating the ssl security on your site so setup an external password. Also this has to be uploaded to a different host that has access to both servers.