Is the question "what is a good free or low-cost tool to EDIT PHP pages" or is it "what is a good free or low-cost tool to manage PHP projects"?
Now I haven't done an assessment of the entire tools market, but the PHPEclipse platform shows promise as a full-featured PHP IDE. While admittedly it may be a bit confusing, I find that once you spend time getting to know its many features you that as a aggregate they are very useful to managing the code as the project becomes more complex. The fact that the platform enjoys broad academic and market support is an added bonus. I may be biased only because I am happy using Eclipse as my Java IDE.
Since the parts come from many different vendors it is also a little confusing to get set up though. I've used my notes to partially documented setting up a PHPEclipse Development environment here. This information may be of interest to those wanting to create new Moodle modules. It is probably too much for those wanting just to create a new theme.