I recently took the test having studied the official exam book and also taking the mock tests. I ended up passing the mock tests by the end yet still failed the exam the first time I sat it.
I am considering whether I should retake the test, but my advice for those looking into it, and who have studied ....
1) Study some more
2) For each function - study HOW they work - not just WHAT they do. One question was - what is the result of count(explode("/","///")) - I knew about count - but not that specific an element. So for each function work with them (it will take longer) and play with them. Learn each of the parameters as mentioned by the manual. For example there is a 2nd parameter to the count function to count recursive - my books didn't mention that!
3) Read through the php.ini file - its not THAT big and will certainly help a lot (these are all steps I'll be doing if I retake the test)
4) Don't rely on the mock tests - they do provide a good sweep of questions, but take the topics they include and then study those indepth as well.
I personally was surprised how hard the exam was having spent many hours studying and working with the language as part of my work. Please don't underestimate how much a real working knowledge of language is really important.