silly question: an IDE for php?

Not to start flame. I have been writing php using just vim for some time, it is a waste of time to put 'echo' statement everywhere to track variable values. I wonder what do the experienced users on php.net use for php development? Do you use IDE? Do you use Zend? Is there an alternative free (free as in 'beer') IDE rather than Zend that I can use to track variables, analize code and so like? I heard of bluefish, is it widely used? Thank you.