First of all, you definitely need to see this Web page, it does a superb, detailed and technical review of three PHP frameworks: CakePHP, CodeIgniter and Yii:
Comparisions of PHP frameworks
(Edit: As a CakePHP veteran I would say the link above is written by someone who skimmed through the manual without spending weeks to learn the framework, so I consider many judgments about CakePHP plain wrong. But then again, maybe that does say something about how a beginner would find the frameworks :p)
I've personally only used CakePHP. Here's some personal opinions based on what I've heard:
CakePHP's looks and feels like a PHP port of Rails. Although since 1.2 there's some significant amount of differences. This basically means the framework is heavily "automagic." As some people have mentioned if you're new to the MVC framework thing you'll probably experience a steep learning curve.
CakePHP's major weakness is its documentation (does it have?) I started Cake during the 1.1 era about a year or two ago. Back then the documentation was non-existent. No joke. It was poorly written and ugly. I remember the official "15 minute blog tutorial" (which was the only one tutorial to read at that time) took me an hour or more to fully comprehend. It was only called 15 minute because the author broke it down into 15 steps.
The situation has improved somehow, the Cookbook gives us a human-readable instruction manual instead of APIs. But if you looked at it you still find many parts are missing once you get past the beginner stage. And due to the Cookbook's wiki nature, I remember spotting several inconsistencies/simply-wrong stuffs in the Cookbook several times this week. Most of the time you need to get some seemingly trivial knowledge by scraping mailing lists or trolling IRC.
CodeIgniter seems to the least strict MVC framework here. Coding it actually feels like writing PHP instead of CakePHP's "following a fairly strict PHP-like relation" (which we can arguably call Domain-Specific-Language).
I haven't looked at CodeIgniter for quite some time. Back when Cake and CI were the big frameworks, CI's documentation was light years ahead of Cake's. They actually had screencasts and serious documentation. All these combined with the fact that CI's approach is more explicit, less magic might make CI's learning curve gentler and more pleasant.
I also remember reading somewhere that CI is the fastest of the major MVC frameworks in PHP, which makes sense because it's less magical approach probably adds less overheads.
Symfony seems to be the strictest Rails port. I haven't used Symfony for a project myself. As far as I know the work-flow copies that of Rails very closely. That means you'll be dealing with YAML files and doing running command lines like
$ php symfony propel-build-model
You can also find command line tools in Cake (they're called "consoles") but they're nowhere near complete as Symfony's. So generally the development/deployment/testing/migration process with Symfony might involve typing command lines, whereas in Cake you would need to create-the-files-manually-by-remembering-magical-file-names/rolling-your-own-migration-by-hardcore-sql/run-the-test-suite-by-running-a-web-page. Well, practically it isn't so bad once you've remembered the stuffs from Cake but Symfony's way feels a lot more advanced. I don't know how CI handles these though.
So to answer your question by "personal opinions", I'd say I prefer Cake to CI because I personally prefer Cake's automagic way. I think that leads to cleaner code and I feel clever writing stuffs in it. On the other hand, Symfony is historically a little bit late to the PHP MVC scene so I haven't had the chance to learn much about it. But it looks like it has matured and gaining some big momentum lately. From what I see it's a very close port of Rails, which is good. I might want to look into Symfony some time later or if someone can give their experiences with Symfony?
Oh by the way:
Zend I don't believe it's an MVC framework. It's notorious for throwing many functions/components into one big clutter.
Pear is a framework?
Yii Framework seems to be a rapidly rising star. Look at this performance benchmark for some interesting stuffs.