errr, i doubt you are running MediaWiki 1.2.0... That is a pretty old version. And I don't know the answer to your problem, but that is very strange that there is code in MediaWiki that uses PHP 5 features like interfaces. Is there a method to put both php 5 and php 4 code in the same file or something? does it run different code if it determines its on 5 with some sorta version check? Anyway, that is a phptal error. Check the php tal site for help
http://phptal.sourceforge.net/