Well, PHP isn't compiled, it's executed.
To write PHP code, you only need note-pad at least, and Zend Studio IDE at most.
There are tons of topics about editors, and which is best. Some I've found useful are:
Komodo IDE
Zend Studio IDE
WeBuilder 2005 (2006 releasing soon)
EditPlus (light & quick)
NotePad++
UltraEdit
I'd really suggest Komodo or Zend Studio as they have the integrated PHP manual so as you type you get the function definitions as a pop-up to help you code properly. But at least try more than one editor, as others are better and worse in ways, and some flat out suck. But when writing PHP it is almost required to have syntax highlighting, which Notepad/Wordpad (Not NotePad++) doesn't have.
It's a personal preference. Search and use... then decide.
I started coding with EditPlus and then found Zend Studio, and way too much money, but found BluMentals WeBuilder and Komodo. Then in searching for a light-weight, portable app I found notepad++. I use mainly Komodo, but for quick edits and such, I use EditPlus. While at work (not my home computer) I use NotePad++.
~Brett