OOP: class varibales question

I ran into a clash between class variables today. I entended a class, and in the new class I created a variable with the same name that the original class defined. This of course corrupted the data in the varible(s). Here's a sample: class A { var $data = "foo"; function dofoo(){ // do something with $data } } class B extends A { var $data = "bar"; function dobar(){ // do something with $data } } Now class A expects it's $data not be messed with by outsiders. Class be (the outsider) doesn't know class A uses $data, and changes it. Or class B calls a method defined in A which uses A's definition of $data, thereby corrupting B's definition of $data. Am I making sense here? I have only read a little about OPP in languages like C++, but I think this is the point of public/private variables? I don't think php supports privates vars, so is there any way to avoid this situation? Thanks, Chris