รบกวนดูโค็ต การตัดสต๊อกสินค้าหน่อยครับ ทำไมมันไม่ตัดให้ครับ

//$sql5 = mysql_query("update product set product_id = balance - '$num[$i]' where order_no = '$product_id[$i]'");

$result = mysql_query("update product set balance  = balance - '$num[$i]' where product_id = '$product_id[$i]'");

		$result = mysql_query("update product set balance  = balance - '$num[$i]' where product_id = '$product_id[$i]'");
		$result1 = mysql_query("update product set balance  = balance - '$num[$i]' where product_id = '$product_id[$i]'");
		$result2 = mysql_query("update product set balance  = balance - '$num[$i]' where product_id = '$product_id[$i]'");

function execute_transactions($arr_sql){
//***** จะใช้ Transactions ได้ Table ต้องใช้ ENGINE แบบ InnoDB ด้วยนะครับ(ไม่ทราบเหมือนกันว่าทำไม่แต่ ENGINE อื่นๆไม่ได้ครับ *****
if(gettype($arr_sql)!="array"){//ต้องส่ง SQL มาเป็นอาเรย์เท่านั้น
echo "<font color=\"red\">Not Array SQL command!</font><br>";
return false;
} else {
global $conn;
$affect = 0;
mysql_query("BEGIN");
for ($i=0;$i<count($arr_sql);$i++){
mysql_query($arr_sql[$i]);
if(mysql_error()){
mysql_query("ROLLBACK"); //ถ้่าำไม่สามารถกระทำได้ต้องคืนค่าเดิมทั้งหมด
return false; //ออกจากฟังก์ชั่นเมื่อคืนค่าเดิมแล้ว
}else{
$affect += mysql_affected_rows($conn);
}
}//for ($i=0;$i<count($arr_sql);$i++){
mysql_query("COMMIT"); 
return $affect;//คืนค่าเป็นจำนวนเร็คคอร์ดที่ถูกกระทำ
}
}//

<?
    	include('conn_mysql.php');
		
		$sql = "select * from product ";
		$query = mysql_db_query($dbname,$sql)or die(mysql_error());
		if(mysql_num_rows($query)>0)
			{ 
    ?>
        <tr>
          <td width="5%"><div align="center"><font color="#FF0000">ลำดับ</font></div></td>
          <td width="29%"><div align="center"><font color="#FF0000">ชื่อสินค้า</font></div></td>
          <td width="16%"><div align="center"><font color="#FF0000">ราคา</font></div></td>
          <td width="17%"><div align="center"><font color="#FF0000">จำนวนที่ต้องการซื้อ</font></div></td>
          <td width="15%"><div align="center"><font color="#FF0000">รวมเงิน</font></div></td>
        </tr>
        <?
		while($rsst=mysql_fetch_object($query))
		{
			$product_id = $rsst-> product_id;
			$product_name = $rsst-> product_name;
			$product_price = $rsst->product_price;
			$product_code = $rsst->product_code;
			$x++;
    ?>
        <tr>
          <td><div align="center">
              <?=$x?>
              <input type="hidden" name="product_id[<?=$x-1;?>]" id="product_id[<?=$x-1;?>]" value="<?=$product_id?>">
          </div></td>
          <td><div align="center">
              <?=$product_name?>
              <input type="hidden" name="product_code[]" id="product_code[]" value="<?=$product_code?>" />
              <input type="hidden" name="balance[]" id="balance[]" value="<?=$balance?>" />
          </div></td>
          <td><div align="center">
              <?=$product_price?>
          </div></td>
          <td><div align="center">
                <input type="text" name="num<?=$product_id?>" id="num<?=$product_id?>" autocomplete="off" onKeyDown=" if(this.value==0) this.value = '';"  onKeyUp="if(this.value==0) this.value=0;document.all.total<?=$product_id?>.value= parseFloat(document.all.num<?=$product_id?>.value)*<?=$product_price?>;" value="0">
              </div></td>
          <td><div align="center">
                <input type="text" name="total<?=$product_id?>" id="total<?=$product_id?>" value="0">
              </div></td>
        </tr>
        <?
		}
		}
			else
		{
			echo "<div align=\"center\"><font color=\"#FF0000\" size=\"2\">ไม่มีข้อมูล</font></div>";
		}
    ?>

<?
	include('conn_mysql.php');
	
	$product_id = $_POST["product_id"];
	$dates = date("Y-m-d");
	$customer_id = $_POST["customer_id"];
	$sql = "insert into tborder (customer_id,pricetotal,dates) VALUES ('$customer_id','44','$dates')";
		$query = mysql_db_query($dbname,$sql)or die(mysql_error());
		
		$sql3="select max(order_no) from order_details";
	$result=mysql_query($sql3);
	$r=mysql_fetch_array($result);
	$order_no=$r[0]+1;
	
	for($i=0;$i<count($product_id);$i++)
	{		
	
		$num = "num".$product_id[$i];
		$num = $_POST["$num"];
		$total = "total".$product_id[$i];
		$total = $_POST["$total"];
		$product_code = $_POST["product_code"];
		$balance = $_POST["balance"];
		 	
		$sql2 = "insert into order_details (order_no,product_id,product_code,quantity) VALUES ('$order_no','$product_id[$i]','$product_code[$i]','$num')";
		$query = mysql_db_query($dbname,$sql2)or die(mysql_error());
		
		$result = mysql_query("update product set balance  = balance - '$num[$i]' where product_id = '$product_id[$i]'");
		
		
		echo "<script>alert('บันทึกข้อมูลเรียบร้อย');</script>";
		}
		echo "<script>window.location='ordercustomer.php';</script>";
	mysql_close();   
?>