On the Zend Developer Zone today there's a new tutorial looking at a database class/component that implements the Nested Set idea for the Zend Framework - NestedSetDbTable.

There are several ways of realizing demand of storing hierarchical data in database, and most popular methods today are Adjacency List and Nested Set models. [...] With Nested Set model [the] whole tree can be retrieved with a single query, but there are lot of things that need to be done when you need to make some changes in hierarchy. From the title of this article, it's obvious which model is my favorite.

The class helps you to manage nested sets of data pulled from a database and extends the Zend_Db_Table component to pull in all of its additional functionality. You define the "left" and "right" columns in the table and can use constants like NEXT_SIBLING and LAST_CHILD to perform inserts and updates without having to worry about a record's place in the set. Code snippets are included to show you how it all works together.