Of course, the reason I haven’t heard of it is because I don’t usually choose Windows as my development platform. In the corporate world, though, I don’t have much choice. PHP Designer integrates really well with many of the *AMP packages for Windows. They recommend XAMPP however I’m certain it would work equally well with WAMP or the others.
Within, it offers syntax highlighting for several languages aside from C, a built-in PHP debugger (you must have the PHP executable installed somewhere for it to use), and several other features. While I wish it offered VI-esque text editing commands, I’ll survive if it feels a little more like wordpad than vi.
I don’t have much PHP IDE Experience to compare PHP Designer to, but I’ve enjoyed working with it for the past 2 weeks. Give it a shot.