[pmwiki-users] changing style by category

[noskule]

hi list
I try to change the style if a specific category exists:

*function* StylePresentation($pagename, &$page, &$new) {
	if (strpos(@$new['text'], '[[!Presentation]]') == *true*) {
		$PageTextStartFmt = "*\n*<div id='presentation'>*\n*";
		}
	};

but dosnt work :-( How do I correctly say: parse the wikitext? 
thanxs for any hints
grz nos