When I went looking for a PHP-based Wiki Wiki Web several weeks ago, there were none so I set off to create one a WikiClone. When I got a reasonable version going and came back here to "register" it, I found two other PHP implementations already there. Sheesh.
Anyway, I based my code on PikiPiki, a Python implementation of Wiki Wiki Web that I admire for its clean design. I have followed the formatting rules of PikiPiki, although I'm thinking about converting to classic Wiki rules.
My biggest goal was to implement a Wiki Wiki Web that allows for very fast browsing. PHP seemed a good candidate since when run as an Apache module we can skip the whole fork/exec overhead thing. Although I'm fairly happy with the result, the algorithm I used to handle formatting chews up a lot of time. If only PHP3 had the '/e' option for regular expressions..
Oh yeah, you can find my code and an example PhikiWiki web via some links on http://www.ontosys.com/reports/PHP.html .