Hi,
    I'm currently writing my own wiki, partly to learn about distributed version control systems. As part of that, I'm implementing a creole parser. 

Its implemented in PHP5.1+, and uses the DOM extension to build the XHTML/HTML of the document.

Its currently running at http://ren.dotgeek.org/creole/index.php .

Wether you get XHTML or HTML depends on wether the useragent explicit states it can accept application/xhtml+xml.