Here's another ... I use HTMLKIT - it has some PHP hightlighting features & is good also for html & css.
SEG is the best after notepad. In both speed and simplicity. It is like syntax coloring added to notepad.
I use Dev-C++ for my PHP coding, although any programming IDE is generally a good coding environment, regardless of the language. The more you're familiar with it, the more efficient you will be when using it.
I like small and light editors so I use Crimson. Since someone mention Programmers Notepad and Notepad Plus, I downloaded and installed these two programs. Found that something is lacking. The syntax highlighting is not as extensive as Crimson's. Say when you type some php function of constant they're not recognized as keywords.
I'm using this exact same combination. I've been using Notepad2 exclusively for a couple of years now. Just recently though, on a couple of recommendations, I've downloaded eclipse and I'm giving that a try. Not sure if I'll stick with it for the long-term yet though.