removing functions?

I'm helping to create a website creation system in PHP, which allows some classes of authors to embed arbitrary PHP code. We do trust our current authors to not stomp on anyone else's toes, but I'd like to disable their access to the underlying system anyway, by disabling filesystem access, system(), dl() and a number of other things once the trusted portion of the script has run (which they have no control over). Basically they'll get a db handle pre-opened for them that lets them manage their persistent data there, and nothing else. I was wondering how I might go about disabling functions at runtime? Or if safe mode can be turned on (and of course not turned back off) at runtime? The script has to do a number of "unsafe" things before handing control over to user code, and pass some of results over to it, so it's not really a simple matter of execing a script that is in a safemode directory. The documentation seems pretty clear on how to add new functions with extensions, but not how to manage existing ones. Is this doable with, say, an extension that meddles with the function table? If so, any pointers on where I should start? chuck (yeah another one)