Calling original function from an overloaded function

I have been looking at xdebug and have figured out how to overload a function. As a test I have overloaded the var_dump function. Now how do I call the original var_dump function from my implementation? Borrowing from OO terminology I have subclassed the var_dump method, now I want to call parent::var_dump() from my subclass. Also how does overloading work when two extensions overload the same function? Can this be done? Is there some sort of order? Can I let the other extension overload the function and then overload the function from the other extension? Any pointers would be greatly appreciated. Thanks Christoph