For a beginner it will be easier Yii. It is simple as cork, but at the same time, with a certain experience, it is possible to write good projects.
Zend — nothing I can say. Probably only it will be easier to move to Symfony. In this respect, the word loses a little Yii — with him to climb harder. I have 2 months to spit when the Symphony moved.
Symfony 2 is probably the coolest PHP framework available today, well, or most promising. I would start immediately, although it will be hard, but more experience.