Written in PHP5, the aptly named PHP Creole Converter is a static class that converts Creole wiki format text into CreoleXML format, and from that to various other formats including HTML.
If you like to use the Twick.it API you should take a look at this wrappers. By simply calling some static methods you can read data from the API and don't have to care about all the REST stuff like sending HTTP request or parsing XML data.