OK, an update on this.
Yes, I am taking ZCE.
I've done one mock exam, and it seems I'd pass, even without reading the study guide. It is interesting to note what areas I'd fail on, but not surprising; I haven't used MySQLi or PDO before, I generally avoid using the streams system, and I tend to avoid functions by reference, which are the principle areas I fell down on.
9 more mocks to go, hoping to have a 'pass' in each area before booking the actual exam.