Rashaud Teague has posted a new tutorial to his blog about data trees and parent/child relationships in PHP applications:
Displaying data in data trees showing parent/child relationships can be important to your application for when users are looking for data. [...] Here I will be showing you (most likely a beginner programmer) how to display data from a database (MySQL) using PHP. In the this example I'm using the data from Maris SimpleDocu, a "simple" documentation system.
His simple table holds data for a series of pages linked by a "parent" key column. Back in the PHP he uses a recursive function to pull in a subset of the data. The end result is output showing the levels of the data, spaced out by dashes.