[Wishlist] whitelisting for functions/classes

[Zhigalin]

disable_functions/classes is a great thing for sandboxing, but actually it is impossible to remember all the dangerous functions/classes.
Maybe you can also introduce whitelisting?
So all the features are disabled, and there is an enable_functions/class directive, maybe also with some predefined set of whitelists for stuff like "echo" and other basic non-dangerous functions?