As a general rule, if you have a choice, I would recommend learning Python (Pylons or Django, I prefer Pylons for it's looser coupling) or Ruby + Rails.

While PHP is used all over the place, if you want to keep your sanity you're better off getting into more...structured systems.

Apart from that, Zend is pretty good. At my last place of employment we went through a bunch of frameworks including Code Igniter, Symfony and Cake. The various members of development all had violent disagreements over all of them. For some reason that I am still unsure about Zend seemed to ... work for us. It was stable, and nobody got into religious arguments over it's implementation. Not sure that's a good sales pitch, but it certainly didn't suck from my view. It added to PHP where it needed to, but didn't require you to learn an entirely different way of doing things. e.g. Lower barrier to entry.