PHP - class question

Hello, To begin.. i'm new to the list... I'm trying to learn good practices in OOP based PHP. I'm actually a bit new to OOP in general, but I've done quite a bit of reading and playing around with it in the past couple of months. What I'm trying to figure out is the proper way to call a class from a class. I dont mean by extending the class, but actaully just getting the functions... For instance... I'm trying to connect to a Database (database class) from my gen_XML class. Do I include the database class in the gen_XML constructor? Do I set a gen_XML variable as a database object (i.e. $this->mydb)? I hope my question makes some sense... Your help is appreciated... Mark Armendariz