For my day job the developers there use Symfony. I've worked quite a bit with it, deploying all the applications and such, and I find it to be a quality system.
It's the only one I have experience with, as I'm not a developer myself. It's another one to look into when deciding which one to choose.
symfony | Web PHP Framework
I don't use frameworks because I like to work direct with PHP rather than an interpreted language version (if that is the right phrase).
It's already a powerful and easy to use language.
Which way to go? It depends on the level of complexity of a new development I guess. I tend to modify existing scripts, write plugins, or create small standalone apps.
If you need to create template-driven sites from scratch then I have seen Smarty being mentioned a lot.
Framework doesn't interpret your code at all. Frameworks handle a lot of the code that you would rewrite for each project.
They provide a framework in which to work so you can get directly down to writing your application.
Dave recently posted that phpLD was moving from Smarty to jQuery.
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If I were going to go with a framework (or change), I think Zend would be high on my list
These guys pretty much invented what we know today as PHP, and they have some of the best in the industry. I visited their office here in Silicon Valley, and it was kind of cool to see some of the people that have made so much possible to everyone.
I go by that area sometimes. I'll have to take a picture of their office sometime.