แนะนำวิธีการคำนวนใน text field หน่อยครับ ต้องการเปลี่ยนค่าอย่างเช่นค่าเดิม 100 = 10000 แต่พอเปลี่ยน 80 = 8000 ทำยังไงครับ

 <?
   
		
   	$loadDetail="select* from (((product inner join stock on product.P_ID = stock.P_ID)
inner join stock1 on product.P_ID = stock1.P_ID) inner join purchase_supplier on product.P_ID = purchase_supplier.P_ID) where purchase_supplier.PS_ID='$PS_ID' group by product.P_ID ";
	$rDetail=mysql_query($loadDetail) or die(mysql_error());
	
	 
		
		
    while($row=mysql_fetch_array($rDetail)){ 
	$qPrice = $row[amount]*$row[P_price];
							$totalPrice += $qPrice;
	$i=$i+1;
	?>
    <tr>
      <td><?=$i?></td>
      <td><?=$row["P_ID"]?></td>
      <input name="P_ID<?=$i?>" type="hidden" value="<?=$row["P_ID"]?>" />
      <td><?=$row["P_name"]?></td>
      <td align="center"><?=$row["amount"]?></td>
      <td align="center"><?=$row[P_price];?></td>
      <td align="center"><?=$qPrice?></td>
      <td align="center">
        <label for="textfield"></label>
        <input name="boat<?=$i?>" type="text" id="textfield" value="<?=$row["amount"]?>" />
      

    </tr>
   
   <? $numItem++;
   } ?>
    <tr>
      <td colspan="3" align="center">&nbsp;</td>
      <td align="center">&nbsp;</td>
      <td align="center">&nbsp;</td>
      <td align="center">&nbsp;</td>
      <td align="center">&nbsp;</td>
    </tr>
    <tr>
      <td colspan="3" align="center">&nbsp;</td>
      <td align="center"><div align="center"><strong>รวม</strong></div>        <div align="center"></div></td>
      <td align="center">&nbsp;</td>
      <td align="center"><?=$totalPrice?></td>
      <td align="center">&nbsp;</td>
    </tr>
  </table>
  <p>&nbsp;</p>
<table width="860" height="195" border="0" align="center">
        <tr>
          <td width="162" height="124">&nbsp;</td>
          <td width="224">&nbsp;</td>
          <td width="228">&nbsp;</td>
          <td width="228">&nbsp;</td>
        </tr>
        <tr>
          <td>&nbsp;</td>
          <td><input type="submit" name="save" id="save" value="Save"  />
          <td><input type=button onClick='window.history.back()' value='Cancel'> </td>
          <td></td>
          <td>&nbsp;</td>
          <td>&nbsp;</td>
        </tr>
        
  </table>