PHP Development in WAMP

I write my PHP code in a WAMPServer-installed environment, running in
Windows Vista (Home Premium). I use Crimson Editor as my code editor.

Running within these constraints, is it feasible/advisable to install
something to work as a PHP IDE? I'm thinking here primarily of the
possibility of line-by-line execution, the setting off breakpoints and
the ability to inspect (and set) variable values on the fly.

If so, what package (free if possible) do you recommend?