Leendert Brouwer has come up with a (complete) look at PHP backends as written for Zend Framework applications.

Some of these frameworks became rather popular, but the big hit in PHP's framework department came when Zend released its own framework - the Zend Framework. Being "the PHP company" and all, their framework got a lot of attention. And it wasn't all marketing buzz. It really is a fairly well architected, functional piece of software that can make the lives of developers easier. But what did the Zend Framework offer us, in reality?

He looks at some of what the framework has to offer - public interfaces, simplified database refactoring, parallel development - and domain objects. The bulk of the post deals with the Service Layer - how to more correctly split out controller functionality from model functionality and separate our the layers accordingly. He throws in a few opinions near the end about things like upfront design, application scale and the possible overhead of his approach.