As much as I love wikis and use PHPWiki I have to agree with troutgirl that there is a loooong way still left to go in the software.
Now try changing any little thing, and note how this results in an instant and information-free fatal error. One of the nice things about PHP is that generally when it fails you get good debugging info... but not with its object system, which simply fatal-errors out. Or check out the fact that Mediawiki actually has two different stylesheet systems, depending on whether you use PHP4 or PHP5 -- more or less totally undocumented, of course. Or note PMWiki, which I couldn't even figure out how to install.