ErfurtWiki is a WikiWikiWeb implementation in the PHP scripting language and mainly contained in one script file. It is PublicDomain (you are free). It was implemented more like a library and thus allows very quick integration into existing web sites, also because it does not enforce a specific page layout or requires you to choose from a few factory page themes/skins.
There are also hundreds of other implementations available. You can find out about them on the large WardsWiki:WikiEngines list or using the WikiFeatures:WikiEngine comparision table (though still incomplete).