Over on the PHKitchen there's a new article in their series of the "best of" various kinds of PHP applications that have been produced, but maybe most people don't know about. This one talks about PHP application frameworks.

An application framework is software that helps you create, quickly and easily, create an application with pre-defined objects and tools/functions provided by the software. For example, you're not creating all of the database functions because the PHP scripts you installed already have a library of them for you to use.

and, of course, the suggested options from this page are listed in the Poll on their main page for you to vote on which you think is the "best of the best" in PHP application frameworks.