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.

If you are an architect with a team of developers, or one developer with a team of managers, here's a list of a set of modules, frameworks and strategies you should have in mind. Not all of them would necessarily apply to you, but if I were you, I would at least think about each one. So if you are starting a new project - lucky you - from scratch, or coming to an old project with a mandate for change, here's a small checklist.

The five types are:

• a good ORM (Object Relational Mapping) layer
• a tiered framework (splitting out interface, business and database logic layers)
• a event-based framework
• a caching framework (or some form of easy-to-use caching)
• an error/tracing framework

Some frameworks, like the Zend Framework, come with most of these already built in, reducing the need for more than one sort of technology stack.