How much "unlearning" when I move to a PHP framework

Not really, it just depends on what/how you've learned something.

It also depends on the particular framework, some add useful functions and tips, while others suggest (or force) you to use different forms of development, filestructures, and functions.

In php, for example, you still can do pre_replace('[A-Za-z]',$user.name), would still work, but is_alphabetical($user.name) would do the same thing.

You still can use the same functions, but some frameworks require you to rework your program's structure, while others just provide helpers, functions, and database classes to use.

I've heard that http://codeigniter.com/ is the most augmenting framework, flexible when it comes to program structure, while http://cakephp.org/ is much more opinionated.

It's not like ruby on rails or django where you would learn a total new language, you still have and can use php, but it's best to stick with good programming principles with the framework, instead of work around them with php.

I think this is why zend and symphony are so good: you can use what you want/need. So you do not have to un-learn anything..
(although after you start you keep adding more and more elements in your app-logic)

That might be true in case you're already using a framework and consider switching to another one. Frameworks tend to differ in the way common tasks are realized.

If you're not using a framework yet, you will "unlearn" only things that are cumbersome and complicated to implement manually. It is supposed to simplify your programming so go ahead and give it a try.