ช่วยดูโค๊ดให้ทีครับแก้ไขยังไงก้ขึ้นว่า Notice: Undefined index: pid in C:\AppServ\www\MyOnlineStore\cart.php on line 39

<form action="cart.php" method="post">

<?php
if($_POST) {
	foreach($_POST as $pid => $qtt) {
		if(!is_numeric($qtt) || $qtt <= 0) {
			continue;
		}
		$sql = "UPDATE cart SET quantity = $qtt
					WHERE pid = $pid AND sid = '$sid';";
		@mysql_query($sql);
	}
}
else if($_GET['pid']) {
	$del = $_GET['pid'];
	$sql = "DELETE FROM cart WHERE pid = $del AND sid = '$sid';";
	mysql_query($sql);
}

$sql = "SELECT * FROM cart
			WHERE sid = '$sid';";
$result = mysql_query($sql);

if(mysql_num_rows($result) == 0) {
	echo "<p align=center>ไม่พบสินค้าในรถเข็น</p>";
	echo "</form></body></html>";
	exit;
}

?>

<table border=1 bordercolor=#cccccc cellpadding=3 align=center style="border-collapse: collapse;">
	<caption>รายการสินค้าในรถเข็น</caption>
<tr bgcolor=#ddddff>
<th align=center width=220>สินค้า</th><th width=50>จำนวน</th><th width=50>ราคา</th><th width=80>รวม</th>
</tr>

<?php
$grand_total = 0;
while($cart = mysql_fetch_array($result)) {
	$sub_total = $cart['quantity'] * $cart['price'] ;
	$pid = $cart['pid'];
	echo "
	<tr valign=top>
		<td>
			{$cart['product_name']}
 		</td>
		<td align=center>
			<input type=text size=3 name=$pid value={$cart['quantity']} />
		</td>
		<td align=center>{$cart['price']}</td>
		<td align=right>$sub_total</td>
		<td><form method=\"get\" action=\"cart.php\">
        <input type=\"hidden\" name=\"pid\" value=\"$pid\" />
        <input type=\"submit\" name=\"pid\" value=\"X\" />
      </form></td>
	</tr>";
	
	$grand_total += $sub_total;
}
?>

<tr>
	<td colspan=3 align=center>รวมทั้งหมด</td>
	<td align=right><?php echo $grand_total ?></td>
</tr>
</table>
<p align=center>

<input type=button value="&laquo;&nbsp;ย้อนกลับ" onclick="location='index.php'" />
&nbsp;&nbsp;&nbsp;&nbsp;

<input type=submit value=คำนวณใหม่ />
&nbsp;&nbsp;&nbsp;&nbsp;

<input type=button value="สั่งซื้อ&nbsp;&raquo;" onclick="location='shipping.php'" />

</p>
</form>