In this new blog entry Marc Gear suggests a few things that you can do to help learn a new framework quickly and easily:

There are dozens of PHP frameworks around now, some attracting more attention than others. I am no expert on these frameworks and have not used a single one extensively so I wouldn't dream of recommending one over the other, nor do I want to enter the debate about what is or is not a framework (I'll work on the assumption that anything calling itself a framework is a framework). Instead I'm going to concentrate on how you can get started with a new framework as quickly as possible.

He suggests four examples that can help you get more familiar with each framework and how they're structured - a "hello world" as a first step, creating a simple calculator, making a guestbook and making a simple script to parse and paginate a remote RSS feed.