Edit: Pack now available from https://leanpub.com/zce
The pack is a single PDF document, including tips for the exam and preparing for it. The ZCE exam is around 70 questions, and the pack includes 70 plausible questions, along with solutions and some explanation of why those are the answers, with links to the manual as appropriate. It's 60+ pages in total and I hope that it will serve to help anyone preparing for the course. These questions are the ones I use in my own courses and I do think that the sample questions are the best way to prepare for the way the computer-assessed questions are formatted. Between the pack I have prepared and the ones in the ZCE study guide from Zend (they're not awesome, but they do give you an idea of what to expect and I promise that the real exam is better!) hopefully there's enough to give you an idea.
Tips for ZCE Preparation
Investigate what is involved – there is a list of syllabus topics at https://www.zend.com/services/certification/php-5-certification/ so take a look there and figure out if that sounds like something you're interested in. Usually I recommend 2 years experience in PHP programming before attempting the ZCE.
Study! As well as my excellent question pack mentioned above, Zend publishes a free study guide for PHP 5.3, with some sample questions in it, so this will give you an idea of which areas you know well and which bits need more study. The guide is linked from the sidebar of the main certification page (it updates, so no direct link)
Research any areas where the questions seemed tricky for you. It's likely that there are some areas of PHP that you haven't been using regularly, so you'll need to study. Also remember that there aren't any reference materials or internet access allowed in the exam, so it might be helpful to make some flashcards or other reminders to help you retain all that information. I curate a bundle of links that I've recommended to ZCE students: http://lornajane.net/zce-links-collection and this is actively maintained so let me know if any of the links are dead or there are resources you'd recommend to others.
Prepare with a course. I've been teaching ZCE for a lot of years now and I rarely teach anyone who doesn't already know a lot of PHP. Running the ZCE preparation classes (I provide these both direct to organisations and as a public course at NTI Leeds, so does Zend, there may be others local to you) gives me a chance to really teach anything which someone hasn't used before, and recap on any tricky questions that come up. I love to teach, but the sheer size of the syllabus makes this easily one of the most challenging classes that comes up for me – and I love it.
Book the exam at a centre near you. If you booked a preparation class, then book the exam very shortly after that, because when else will you spend a number of days uninterrupted just thinking about ZCE topics? Don't let confidence put you off.
Practice with some questions and make sure you know what to expect in the exam itself (more tips in the questions pack and in Zend's study guide).
Most of all - good luck!