The first version ran under PHP 3.x and ran on DBM files only. It was a feature-for-feature reimplementation of the original WikiWikiWeb at c2.com.
In early 2000 Arno Hollosi contributed a second database library to run PhpWiki on MySQL; from then on the features and contributions started to pile up, including a templating system, color diffs, rewrites of the rendering engine and much more. Arno was interested in running a wiki for the game Go. http://senseis.xmp.net/?GoWiki
Jeff Dairiki was the next major contributor, and soon headed the project for the next couple of years. His interest was in Hammond Organs, and he ran a wiki for Hammonds: http://www.dairiki.org/HammondWiki/
Reini Urban, Carsten Klapp, and Joel Uckerman are the primary developers today and PhpWiki is still maintained and developed.