phpscript as command line with setuid, weird result

Hi, I wrote a command line script which makes some administration things on my server. The script is setuid (set uid to root). I call the script simply through its name. I've put a >>print getuid()." ".get_current_user();<<< in the script to see under which user it is running. When I su to nobody and run the script it returns "0 root". apparently it has got root rights at that moment. But the script can't execute anything it should be able to, example: creating a file in root's directory, cat-ing the /etc/shadow.. It always says: "No permission etc" Is it a security switch in the php.ini or what can this be? thanks in advance patric de waha