I think the webserver do have the permission.
Actually, what i want to do is to upload a .tar file and untar it at the same time.
$temp=$HTTP_POST_FILES['file']['tmp_name']
if ($file)
copy($temp, "/sid/htdocs/temp/upload.tar");
//since now, every thing is fine. The file is successfully uploaded.
exec("/bin/tar -xf /sid/htdocs/temp/upload.tar");
// the exec part seems not working at all
so, I really don't know what's wrong with it.