Using object oriented PHP to display users in a database
Some tips which might help you:


Make as few static methods as possible. Those functions should not
appear in any other function as they get in a way in unit tests. I usually use them to spit out html in views.
Don't use global varibales.
Hide as implementation as you can. Make public only those things that are vital for functionality.
I consider new operator harmful as it cannot be mocked. Use dependency injection instead to provide object with all necessary data.


Last but not least I highly recommend book Clean Code by Robert C. Martin if you care about keeping your code clean.