On NetTuts.com there's a new article posted that introduces you to the concept of a persistence layer in a PHP application:

One of the most confusing design pattern is persistence. The need for an application to persist its internal state and data is so tremendous that there are likely tens – if not hundreds – of different technologies to address this single problem. Unfortunately, no technology is a magic bullet. [...] In this tutorial, I will teach you some best practices to help you determine which approach to take, when working on future applications. I will briefly discuss some high level design concerns and principles, followed by a more detailed view on the Active Record design pattern, combined with a few words about the Table Data Gateway design pattern.

Included in the post is a high-level application design with the business logic is at the core and the persistence technology/layer exists outside of it. They show how to create a simple, working solution for a persistence layer to handle a blog post and its contents. It talks about characterization tests, the table gateway design pattern and the possible move to the active record pattern.