Are you referring to PHP's "safe_mode" function?
If so, you have to edit your php.ini file and find the line that says:
safe_mode = Off
Change that to say
safe_mode = On
And then restart Apache. There are some other related settings in php.ini that you may want to check as well - such as safe_mode_gid, safe_mode_include_dir, safe_mode_exec_dir, and safe_mode_allowed_env_vars. You should be able to find more information about these and more at PHP's web site:
http://www.php.net