In this new post to his blog Jani Hartikainen looks at implementing the Data Access Object pattern in your PHP applications.
The advantage of this is that you can easily implement different methods to persist objects without having to rewrite parts of your code. I'm again going to use the programming language quiz game I wrote as an example. Since I initially wrote it to use Doctrine ORM directly, and both the old and new code are available, you can easily see how the code was improved.
He starts off with a look at the pattern itself (including a diagram of how an example would work with Doctrine) followed by the creation of the models for his Questions example. Add in the factory to create an instance and an exmaple of it in action and you're there.