Not sure how many people realize it, but when you start dealing with multiple users access a system (in this case a web server), you have to be very careful what you store in static's (global variables).  This is because all the users will have access to this data and so if you create a page, you could allow them to see data that they shouldn't.

The main place this comes into play is with SQL data.  We recommend that you cache data from the database so you don't have to query for it on each request.  But if that data is sensitive, then you need to make sure you do not store it in cache or some other global.