well there are multiple answers, honestly its really personal preference. anything with syntax highlighting is recommended. [-link-] <- this thread will help. notepad++ seems to be a popular choice.
Generally, devs are moving towards Aptana (free). Personally, I'm still using Ultra Edit Studio (paid for, lifetime license). But as rgk says, personal preference mainly.
Thanks Bugrain, checking out Aptana today, also found Delphi for PHP, but earlier experiences with Delphi leave me feeling like I will need a rubber suit....
Worth looking at Aptana if you're going to do relatively complex stuff. For the simple stuff, a decent editor is probably sufficient (and less of a learning curve)
Actually Eclipse + PDT extension is exactly what you'd need if you go for all IDE goodies. Aptana just automates somewhat the installation process and offers some more features (mostly non-php related). Zend studio is the best php IDE IMHO, but costs money...
This topic is some months old but I'd like to inform people about an editor that I have been using recently: Sublime Text 2. It's not an IDE but is has great features and even more amazing performance (in terms of speed and stability). It comes with a huge amount of features that are similar to those in the software mentioned above.
I'd say give it a try, it has a free trial (no time restriction or any restriction for that matter)!