chown/chgrp not working

I have a client that I made a image uploader/thumbnailer for that insists on using FrontPage. In order to make FP happy, I need to set the owner to 'nobody' and the group to 'site3' which are uid 99 and gid 117. The uploaded file is set as httpd/root and the thumbnail created by the script comes out httpd/site3. First I tried: chown($file_name, 99); chgrp($file_name, 117); Next I tried: chown($file_name, '99'); chgrp($file_name, '117'); This fails to get the uid or gid, so the first syntax appears to be correct. Finally I tried: chown($file_name, 'nobody'); chgrp($file_name, 'site3'); Error message for first and third attempts is identical: Warning: chown failed: Operation not permitted in /home/sites/site3/web/dev/maintenance.php on line 191 Why is it 'not permitted' and how do I allow it? -- Paul