PHP和SQL中的未定义变量

Question

为什么在以下定义所有变量时会得到未定义的变量错误，例如'client_id'和'order_limit'等？

if (isset($id)) {
$sql = ("SELECT * FROM client WHERE client_id = '".$_GET["id"]."'");
    $result = mysql_query($sql);

if ($row = mysql_fetch_assoc($result)) {
    $client_id = $row["client_id"];
    $client_name = htmlentities($row["client_name"]);
    $order_limit = number_format($row["order_limit"]);
    $notes = htmlentities($row["notes"]);
} else {

    die();
}
}