PurpleTree is written in a few hundred lines of PHP. It uses ParseDown to render the MarkDown to HTML. In addition to ParseDown, it converts WikiWord's to links, along with a few other things, SuchAsAbusingLinkNotationToProtectWikiWordsFromBeingTurnedIntoLinks. (The definitive guide is the PtWikiRendererParsedown1Php.)
Syntax highlighting is done via highlight.js.