If I were to learn a open-source PHP framework, which one should I learn? Seem to me that Zend is most popular. But Yii is regarded highly. Thoughts?
I have used the Zend Framework 1.X extensively. The framework is complete, meaning nearly every useful web app action has a library to support it. I would say that the framework has a higher learning curve than others. However, with the Zend Framework 2.0 coming out soon (2.0 beta was recently released), it might be worth waiting for the new version which is supposed to be simpler and more performant. The strongest point with Zend is that its not going anywhere and most likely over time it will only gain in market share.
Symfony2 is another popular framework that I have used and liked. Never had any experience with Yii.