Ask 20 programmers and you will get 19 different answers on that question.
You should try several different editors to find the one you like the most.
I use Notepad++
with some plugins as my editor with basic functionality
I have used Webocton Scriptly
before, which is great too.
Try UltraEdit and Textpad, too. Both are well known.
vim is a little bit different, but if you really like keyboard navigation it can be the fastest editor you can use.
If you search for a full IDE, there are 3 i can recommend:Eclipse PDTNetbeans PHP
(which i actually use)Zend Studio
(expensive, but maybe the best IDE in combination with Zend Server)