Brushing up

I'm an avid PHP user however in learning PHP I never learned about some things, classes, sessions, and accessing mysql or any database for that matter (and obviously haven't learned about anything I don't know about at all.)

Where/how would be a good way to learn about those, and what are they good for? I pretty much just program as basic as I can, so which instances would sessions be good for/necessary, same for classes.

Thanks :)