Extension problem

I run a MediaWiki-based site, and I am working on writing an  
extension, of the type that adds an XML-style tag which is processed  
by a callback. On first inspection it works fine, but if a page  
includes multiple templates, each of which contains an instance of  
the extension's tag, then only the last one displays correctly, and  
all the others appear as a long string starting with UNIQ and  
followed by mostly-random-looking data (but also containing the tag's  
name in the middle).

I can only find a brief reference to this issue (or a similar- 
sounding one), at <http://meta.wikimedia.org/wiki/ 
MediaWiki_extensions_FAQ#Templates_with_extension_tags>, but I don't  
understand what the fix is. I'm running the latest versions of  
MediaWiki and PHP. What am I doing wrong?