Homesite and it's cousins ColdFusion/JRun Studio all work nicely in a Win32 environment. There is also TextPad and UltraEdit, both of which are excellent.
Most of the Win32 editors that are specifically for PHP are immature. There are exceptions such as NuSphere's but I'm not dropping $300 for functionality that I can do in Vi.(and already own CF Studio at the office).
Under X windows there is also Quanta.Their commercial "gold" version for Win32 might be worth looking into. It's a HomeSite clone for the most part, though not as functional yet. If you plan to spend $ HomeSite is hard to beat.
In a mixed environment (Linux/*nix & Win32) then bouncing between HomeSite and Quanta is pretty straigh forward.