how to safely eval user-generated code

Hi,

I realize that eval() is generally discouraged, but I've found myself
wishing that I could execute user-generated code.

One idea I've seen is to use token_get_all(), and then make sure no
T_STRING tokens match known "dangerous" function names.

Where could I find such a list of "dangerous" functions? What are the
pitfalls of this approach? Is there any way to safely allow user-
controlled scripting, or is it just a bad idea in general? Thanks.

-Emmett
.