safe mode not working

I've enabled safe mode on my local test server, but it doesn't seem to be working. If I run a script owned by one user (me), and within it include (using include()) another script or file owned by another user, the include is successful, whereas it shouldn't be in safe mode. I can verify through phpinfo() that safe_mode is on. And the problem occurs whether safe_mode_include_dir remains unset or set to an empty directory. Any ideas? Thanks, --Dave