Query7.com: Why You Should Be Using A PHP Framework
On the Query7.com blog, Logan has posted his opinion on how you should be doing your development on sites that are more than just one or two pages - you should be using a framework.
Frameworks should be used when constructing web applications. Any application that involves a database, forms, sessions, cookies or a remote service (such as Twitter or Facebook) will benefit from being powered by a framework. There is no need to use a framework for a website that has only one or two pages, nor for command line utility scripts.
He lists some of the common features frameworks provide including database abstraction, caching, form management, authentication and internationalization. He also includes some of the more general benefits you get from using frameworks like portability, shorter development time, application security, plugins/module support and the enforcement of good coding standards (depends on the framework, obviously).