经过多次尝试但总数不显示

Question

我无法找到错误，你会帮我解决这个问题吗

<tfoot>
        <tr>
            <td>
                <?php
                $total = '';
                $s = mysql_fetch_object(mysql_query("SELECT SUM(amount) as amount FROM tborderdetail 
                WHERE order_id = '$id' ")) ;
                $total = $s -> amount;


                ?>
                 Today :  <?= $total ?>  $
            </td>
        </tr>

    </tfoot>