Thread:Project:Support desk/the class "mktree" doesnt work

I see that in our old wiki the class "mktree" would convert the list below it to a tree which had + signs to close and open.

But the same class "mktree" doesnt do anything when I upgraded to php 5.3 and mediawiki 1.20 (on a different server). I also upgraded the Extension CategoryTree

Where should I start looking to see why this class no longer has any effect?

Thank you, Diana