รบกวนช่วย code ระบบตัดสต็อกให้ผมทีคับ ผมจนปัญญาจิงๆคับ

<?
	session_start();
	include('config.inc.php');
	session_register("cart");
	if($_SESSION['ID_User'] == "")
	{
		echo "<script language='javascript'>alert('Please Login');history.back();</script>";
		exit();
	}
	

	$strSQL = "SELECT * FROM register WHERE ID_User = '".$_SESSION['ID_User']."' ";
	$objQuery = mysql_query($strSQL);
	$objResult = mysql_fetch_array($objQuery);
	
	if($_REQUEST['act']=='add' && !empty($_REQUEST['ID_Product']))
	{
		if(isset($_SESSION['cart'][$_REQUEST['ID_Product']]))
		{
			$_SESSION['cart'][$_REQUEST['ID_Product']]++;
		}
		else
		{
			$_SESSION['cart'][$_REQUEST['ID_Product']]=1;
		}
	}
	
	if($_REQUEST['act']=='remove' && !empty($_REQUEST['ID_Product']))
	{
		unset($_SESSION['cart'][$_REQUEST['ID_Product']]);
	}
	
	if($_REQUEST['act']=='update')
	{
		foreach($_POST['qty'] as $_SESSION['ID_Product']=>$_SESSION['qty'])
		{
			$_SESSION['cart'][$_SESSION['ID_Product']]=$_SESSION['qty'];
		}
	}
?>

	<form id="frmcart" name="frmcart" method="post" action="?act=update">
  <table width="600" border="1" align="center">
  <tr>
  <td>Cart</td>
  </tr>
  <tr>
  <td>Product</td>
  <td width="117">Price</td>
  <td width="25">Unit</td>
  <td width="42">include</td>
  <td width="51">Remove</td>
  </tr>
  
 <?
 	$total=0;
	if(!empty($_SESSION['cart']))
	{
		foreach($_SESSION['cart'] as $_SESSION['ID_Product']=>$_SESSION['qty'])
		{
		$sql = "SELECT * FROM product where ID_Product = '".$_SESSION['ID_Product']."' ";
		$query = mysql_query($sql) or die ("error=$sql");
		$objResult = mysql_fetch_array($query);
		$_SESSION['P_Price'] = $objResult['P_Price'];
		$sum = $_SESSION['P_Price']*$_SESSION['qty'];
		$_SESSION['total'] += $sum;
  ?>
  <tr>
  	<td width="334"><?=$objResult["P_name"]?></td>
    <td width="46" align="right"><?=$_SESSION['P_Price']?></td>
    <td width="57" align="right">
    <input type="text" name="qty[<?=$_SESSION['ID_Product']?>]" value="<?=$_SESSION['qty']?>" size="2" />
    </td>
    <td width="93" align="right"><?=$sum?></td>
    <td><a href="order.php?ID_Product=<?=$_SESSION['ID_Product']?>&act=remove">remove</td>
   </tr>
   
   <?
   }
   ?>
   <tr>
   	<td>total</td>
    <td><?=$_SESSION['total']?></td>
   </tr>
   <tr>
   	<td><a href="user_page.php?id=2">BACK TO PRODUCT</a></td>
    <td>
    <input type="submit" name="button" id="button" value="update" />
    <input type="button" name="submit2" value="Checkout" onclick="window.location='order3.php';" />
    </td>
   </tr>
   
  <?
  }else
  {
  echo "NO CART";
  }
  ?>
  </table>
  </form>