> What is the most obvious way to implement a tree-like structure in PHP?

Use someone elses code that already works. :) Array based tree class is
here:

http://www.phpguru.org/Tree/Tree.phps

--
Richard Heyes
+44 (0)800 0213 172
http://www.websupportsolutions.co.uk

Knowledge Base and HelpDesk software
that can cut the cost of online support