Before you select a PHP framework, there are a couple of things you need to know/choose from in order to select the best platform:
- Are you going to run on PHP4 or PHP5?
Not all PHP frameworks supports both PHP4 and PHP5. Some of the framework rely on PHP5's object oriented additions and that mainly affects the developers themselves, whetehr they know/want to develop in an object oriented manner.
- Some frameworks are more sophisitcated then others which sometimes mean the learning curve is a lot higher. You need to know what the web site you need to develop will do and what load is expected on it. If this is just for experimentations, that's ok, but if this is a full blown production site with 100,000 hits per hour, you need to know what you are up against to tailor the best solution.
After you'll answer that you can take a look in this article comparing 10 of the most popular PHP frameworks.
In addition that, I would recommend reading about the MVC (Model-View-Controller) pattern, which is a common design pattern used by some of the most popular PHP frameworks as well as read a bit about ORM (Object Relational Mapping) which is a technique to store objects into relational databases which is also a pattern some of the common PHP frameworks use.
Some of the newer frameworks (such as Cake) are based around Ruby-On-Rails which is a web framework based on the Ruby langauge that provides a complete stack (include the client side) to build web sites that utilize the AJAX technique, so you might want to dig closer into some of the frameworks that take this approach to make the development of AJAXian applications easier (Ruby On Rails uses the MVC design pattern).
If you have more specific questions about specific frameworks, feel free to ask them here, in your very own Q&A blog 
