Debugger to follow control flow and namespaces with

I can already see that I'm going to need a PHP debugger since I can print variables out to the Joomla webpage. It will also show what namespaces are defined in any given part of a program and also show me control flow.

Wikipedia has a list of PHP editors and debugger front-ends.

What debugger to choose?

I downloaded Activestate's Komodo IDE to check out first.