Constants inside classes

Folks, This is really an object-newbie question, but it relates to design of PHP. I have figured out that it is not legal to DEFINE a constant inside a class - a constant must be global. One can have the same effect by creating a var inside the class that one doesn't modify, of course. But that's not quite the same thing (it requires that teensy amount of discipline.) Why? Is this because the DEFINEd constants are more static than classes in some way? G