Mayhaps the script in question is running as a CGI, under Apache? In which case you might want to check out suEXEC (other web servers have similar mechanisms, IIRC). I, too, wonder why you would need the sort of scheme you're describing unless it were something like this (if the user runs the script, the script can do anything the user has permissions to do).

Philosophy can be made out of anything. Or less -- Jerry A. Fodor