The NETTUTS.com site has some advice on how you can add a bit more power and flexibility to your PHP applications - shift them over to a multi-tier application structure.
As PHP applications become more and more complex, it can be easy to end up with a tangled mess of code that makes maintenance nearly impossible. Applying the concept of tiered applications can help to alleviate some of the difficulty in maintaining complex applications.
Tiered programming is a method that splits up the components of an application (like a separation between presentation and business logic). They talks about what a setup like that might gain you, the wrong way to do it and the right way that splits it out into the database tier, the business tier and the presentation tier.