DevShed has posted a series of tutorials talking about different sorts of service layers in PHP applications - seven of them to be exact:

If you’re looking for an approachable guide that teaches you how to implement an easily-customizable service layer in PHP, then take a peek at this article series. In a step-by-step fashion, it walks you through the development of a sample web application, which uses a service to perform CRUD operations on a domain model composed of a few user entities.

Service layer types covered in the series are:

• Working with database entities
• Data Mappers
• Database adapters
• Handling Entity Collections
• User Services
• Dependency Injections
• Modeling Domain Objects