I like to use PHP Designer 7 as a personal preference, however, if you want something free, then I would recommend Eclipse.
I've only used regular notepad, vi in ssh, cpanel's code editor and direct admin's editor... I've never used an external program for my PHP... Maybe I should lol
ZendStudio used to be great, but now is very slow as it's based on Eclipse. Use PhpDesigner for Windows and Komodo if on Mac OS X
Notepad++ is easy to use and free.. and of course you get dreamweaver for free.. where exactly did you check out?
It's all about taste when it comes down to that. Are you looking for a regular editor then crimson, notepad ++, sublime text would suite your needs just fine, even supporting syntax highlighting. If you would like an IDE to autocomplete statements, end brackets etc, then I strongly recommend Eclipse or netbeans. My personal favorite is Eclipse, but that's because I know the shortcuts there, which I haven't studied in netbeans. A little warning on notepad ++: My friends and I, have been using it for some time, and it happens that when you copy/paste stuff into it, it might make some invisible characters, making your code useless.