function chown($filename, $uid, $recursive = false)
{
// quoting from <http://www.kernel.org/doc/man-pages/online/pages/man2/chown.2.html>,
// "if the owner or group is specified as -1, then that ID is not changed"
$attr = pack('N3', NET_SFTP_ATTR_UIDGID, $uid, -1);
return $this->_setstat($filename, $attr, $recursive);
}