Best way to code an online calendar/agenda?

Hey guys, I've been working on a summer project with a couple of friends and I need some help in figuring out what are the best ways of acomplishing what we are trying to do.

We are trying to make a sort of calendar/agenda application that you can use online. Rather than making an app that we distrubute and people can host on their own site, we'd like to host it ourselves and provide subscriptions. Aside from storing the info that the users enter, we have a global database of events/activities that users can search to add automatically into their entry. Users will also be able to create events/activities that they can save into their own database for quick use later on.

What I'm having trouble with is how this sort of system is best implemented. We've got the main database and the general transactions users can do coded in PHP/MySQL so I need to stick with this software duo. I have a clear idea of how everything would work if I have only one user (Use one database for everything basically). However, what's the best way to setup users with their own database, profile, etc.

I've reasearched a lot online but haven't been able to find best/standard practices when it comes to subscription based apps. Would I be better off making a global user database, where everyone gets their own table, or how about giving each user that subscribes a database of their own (and access to the global database). I'd be very grateful if anyone could point me in the right direction. I've got a lot of code to write and it would suck if I had to rewrite because the best implementation does not match up with my code so far. Thanks.

I have done something similar but a little more long winded. I have produced web software on subscription for specific industries. I have found, that apart from it being difficult to persuade someone to share their information in the same file as another user, (especially when they are paying for the priviledge) it is probably better to use different databases for each user.

I have one admin database, and each customer has an identical database with all the same tables, but different data obviously.

It does mean connecting to multiple database in some scripts but with some careful design it is possible to design around this.