ارور Undefined variable

[MMSHFE]

if (isset($_POST['username'], $_POST['password'], $_POST['rd'], $_SESSION['idd'])) { 

    $_SESSION['idd'] = $_POST['username'];

    $n = $_POST['username'];

    $p = $_POST['password'];

    $id = $_POST['rd'];

    $db = mysql_select_db('dbeshop');

    $in_sql = "SELECT * FROM `customer` WHERE (`user`='{$n}' AND `pass`='{$p}' AND `taid`='1')";

    $result = mysql_query($in_sql);

}