将查询更改为

$result= mysql_query("Select id, item, amount, (item * amount)  total
 from table_name");

然后进入while循环

then in while loop

while($row = mysql_fetch_array($result))
    {
       echo "<tr>";
       echo "<td >" . $row['id'] . "</td>";
       echo "<td>" . $row['item'] . "</td>";
       echo "<td>" . $row['amount'] . "</td>";
       echo "<td>" .($row['total']  "</td>";
       echo "</tr>";
     }