Just finished the Zend PHP5 Certification exam, and passed. That makes me one of about 750 Zend Certified Engineers with both PHP5 and Zend Framework certification in the world.
Without giving anything away, the best thing you can do for yourself is look at every module that you will be tested on and thoroughly read the PHP manual. The practice tests are also definitely worth the money. A lot of the questions on the test are simpler than what they ask you, but the real thing definitely threw me a few curve balls.
I'll echo something I've said before about how useful flash cards and note-taking is when studying for exams like this. I also took notes during the practice exam this time (remember, there is no practice exam yet for Zend Framework Certification) on things I wasn't familiar enough with. I feel like the experience was very worthwhile. Not only did I complete something I set out to accomplish, but I improved my knowledge of PHP along with it.
If you've trained yourself well, you should be more confident than normal at the core functionalities of PHP, and not so dependent on the manual, which is a common pathology for the average PHP hacker.