I would recommend phpDesigner 2008
http://www.mpsoftware.dk,
it has inline debugging + loads of other features, compared to other ones i
tried before, such as RapidPHP which looks good on surface but kept crashing
and has loads of bugs when I started using it, also I tested VS.PHP which
is a php addin for visual studio
http://www.jcxsoftware.com/vs.php since I do c# as my day job i
thought it would be great but noticed it had quite a few bugs and limited
features and support + it's more expensive than others.
Lastly if you don't want to spend any money there is always Notepad++
http://notepad-plus.sourceforge.net/uk/site.htm .