It is based on the example editor found in chapter 10 of "Beginning PHP4" from Wrox.The original code is available online here.
I have changed it around to use iframes instead of proper frames. I've also edited out some of the featuresthat I don't want or need. Finally, I've included a stylesheet similar to the one for this site. The result is a much more polished and slightly leaner editor. As I spend more time using the editor, I find that it resembles the original less and less. Their version was a nice base.
WARNING: Please be sure to put this script somewhere password protected! Otherwise, strangers might edit your pages. I make no promises, you have been warned.