Depends on what you need it for. Different frameworks handle different situations better than others. If you haven't had the need for a framework yet, then perhaps you shouldn't bother too much with looking. I generally start reaching for tools when I feel that what I'm currently using could be improved on.
Most PHP frameworks are just some sort of approach to the Model-View-Controller pattern and a collection of libraries for helping with typical things that you might deal with in web development. You can mix and match libraries from different frameworks and they all have their different methods of usage.
I would probably go with something simple, lightweight and well documented. Some frameworks have so many steps and focus stealing side shows that it's a project just to get started with the things.
Maybe take a look at Fatfree (http://bcosca.github.com/fatfree/) and then expand into other frameworks from there if it doesn't do what you need it to.