I'm run a PHP development shop and have interviewed well over 100 people for PHP development positions over the last year or so. To me, it's important that someone knows PHP more so than a particular framework.

At our shop, we use Zend Framework for some projects and homegrown "frameworks" for others. Most decent frameworks will have concepts that will transfer to other frameworks. MVC would be the biggest one that's good to have experience with, whether that's through Yii or another framework. It's also important to just learn PHP on its own. A developer who cannot code without a framework is less valuable to me than someone who knows PHP but doesn't know any framework.

Yes, there is knowledge you can map from other frameworks

Such as (H)MVC, ORM, Templating... not to mention other more intricate stuff, such as DIC (Dependency Injection Containers).

What you would want is use more than one PHP framework so that the person that is thinking about hiring you wont doubt you can learn the framework they use... and maybe even teach them a thing or two (e.g. TDD, Symfony and other frameworks have PHPUnit integration).

At least they'll know you know

Also, I'd personally say that pure-PHP developers are looked down upon. Knowing a framework kind of hints that you at least know OOP and maybe other best/real-world practices.

The classic issue that I see with frameworks is that sometimes there is a reliance on them to accomplish tasks without a full understanding of what exactly the framework is doing.

To a shop that doesn't use a framework, you may struggle working through code and classes that are done differently in a framework. This doesn't matter if you're a proficient PHP developer - you should see existing code and understand it, even if it isn't abstracted through something like a framework.

This isn't restricted solely to PHP, either - I see the same issues with jQuery constantly. There are developers that don't care to know how this magical function works, and their code performance suffers in a big way.

Also, no-framework development is very common. I remember personally not seeing frameworks become popular for about 4-6 years after I started developing web applications. I imagine there are still quite a few webapps in the world that haven't made the switch yet.

Speaking from my experience, in which I'm in charge of a development team in one of those huge companies from the corporate world, and I run technical interviews every now and then, you don't have to worry too much about it...

As long as you are using a framework that was not 'invented' by your company (in case it's a small dev shop, not freaking google), you are in good hands in terms of using an open source framework. Jumping from Yii, to CodeIgniter , to Zend is not that hard, and when I make technical interviews i always take into consideration if they use any frameworks in their daily work, that's right: 'any'. On the other hand, someone who comes to me looking for a job and it's not using any framework, that's a -1 right there, just like it's a -1 for someone to say they love so much their framework they wouldn't consider something else, or even worse, to work with bareboned-PHP.

Just like dukeofgaming mentions, at least you'll know some concepts like ORM, OOP, etc that will come handy in other frameworks and even other programming languages. However, i always ask possible candidates if they know why their are using that specific framework, ask yourself that or to the senior members of your team, someone must have taken that decision and they probably have a compelling reason to do so (or perhaps the decision was taken after a few drinks at the local pub...).

As long as you stay out of hacks, hybrid implementations (half done with Yii and half with barebone PHP, with bits of CodeIgniter) you are in good hands learning Yii... Just remember there are other things out there and keep the learning curve steep, because at some point or another, Yii won't be enough.

Every employer will give extra points to prospective employee if they were using that exact framework or programming language for X years thats for sure, but take into consideration that technology is changing at such a fast pace that companies cant look for employees with 3 years of expirience since that technology is often a year old.

Few years ago CakePHP & CodeIgniter were the most popular FW's in PHP, then came Zend & Symfony, now Symfony 2 and Yii are most popular ones(I might be wrong about exact order but that is not the point here) and next year most popular framework will be something new. If you decide to change company next year there is a real chance that you will be applying for a position using FW or programming language that is not invented at this moment or not widely known at all.

You should treat FW/programming language only as a tool for getting things done, if you are smart and have skills you will have no problem to adopt to new tools quickly and a lot of employers dont test your knowledge of using a tool but your ability to think.