Isolated Execution Environment in PHP? (a la Safe module in Perl)

Hi, We've created our own CMS in PHP and we'd like to allow our users to do more sophisticated things, like embed there own PHP code in pages. We already run in safe-mode with our code, but we would like to run their code in an even more restricted environment than our own code (ie, disable some more functions, etc). Something similar to Perl's Safe module (http://www.cs.usask.ca/resources/documentation/perl/Safe.pm.html). Is this at all possible in PHP? Can you turn on more safe mode restrictions on certain bits of code? Thanks in advance. -- Eric Dorland eric.dorland@mcgill.ca WCG 514.398-5023 ext. 09562