I want the event calendar on my site to work with the Outlook calendar, so when the users add events to the online calendar, they also can be added to the Outlook calendar (synchronize). Obviously, a site built using PHP can't integrate with a desktop application directly, so I am looking for alternative options.
Some options I found:
Using PHP ICAL library, that creates an .ics file and the user can download it and install. Not the most user friendly way of doing it, but might work;
Use SharePoint or Exchange server, but have no previous experience working with Microsoft technologies, so I don't really want to use this option;
Use Google Calendar and Zend Library (gdata), create online calendar and add events to that. Then, using Google Calendar Sync tool, the user can sync the online calendar to Outlook. But the problem here is the all users need a Gmail account to use the Sync tool—not ideal when you have thousands of users on the site. Unless I create one global Gmail account and create many calendars on that account, if that's even possible.
So my questions are:
Are there any Outlook plug-ins—paid or free (ideally)—that can achieve this?
Are there any web services / APIs that can achieve this?
Do you have any other ideas or solutions?