There are a growing number of Wikis written in PHP, the PhpHypertextPreprocessor scripting language. See the PHP section in WikiEngines for more.
Here is a list of "active" PHP WikiWiki sites as of June 28, 2003:
PhpWiki (1MB+ download with 600 files, GPL, flat files, should be comprehensive but wow, 600 files?).
WikkiTikkiTavi (29K & 40 files, GPL, many reference links, the latest is version .25 on Source Forge MySQL. Really nice). The instructions are sparse, but there a lot of sites using it.
PhikiWiki (Small 6-8 program files, flat files, but it wanted to write outside of it's directory -- which I don't allow, but their demo did look nice).
CoTeia (Links off to a site in Brazil (.br) & is in Portuguese).