Judging from my previous experience with interviewing candidates: "Being able to install Wordpress is enough for some people to call themselfs php programmer"
More Serious:
From what I'm currently hearing at conferences the time of "plain old php" is passing and most people are searching for Software developers that have experience with $Framework-X (e.g. Zend Framework, CakePHP (some) or Symfony).
If you'd apply for an entry level position I would expect you to know the basics about OOP, having looked into some bigger framework, done some work with php (any type of project will do) and that our desire to learn.
The Value of an Zend Certified PHP Developer, for me personally, only lies in the fact that we can skip the "is he/she a complete moron that knows nothing about programming at all" questions. While beeing interested in stuff like Test-Driven-Development (or testing in general) is also a plus at the moment. (I'd say, it's where php seems to be at the moment / it's "in")
Hoped that helped a little
Reply to the comment
'the time of "plain old php" is passing' - so now I have a whole extra layer between me and getting a job, just because I rolled my own framework? What's the learning curve like on these frameworks? – Kristina
Stay positive :) There are many people looking for motivated PHP Developers at the moment, depending on where you live of course. I just know about a few countries ;)
Many companies that are now starting more and more php projects (or do so for a year or two) usually don't care if you can write your own caching layer or a database connection handler. Those things are already build and people care about your ability to use something that already exists and just code the parts of an application that earns you money ;)
Having rolled your own framework was, hopefully, a big learning experience and if you find a big that deals with a lot of legacy code it's even more useful. Also you need to be able to wire all these components together etc.
For the learning curve: Getting the hang of an competent framework like Zend Framework isn't a big problem.
The only "bigger" part is the MVC Component but we are talking days, not month or anything, to get something going. Starting with a "real" big framework like Symfony shouldn't be too hard ether since there are lots of resources available on the net.
Imho a company that hires you shouldn't care too much about the fact if you know the framework THEY are using, the people there will get you up to speed rather quickly anyways. If you seen one you've seen the basics of them all