ผมได้ลองทำตามคำแนะนำของสมาชิกในบอร์ด แต่ข้อมูลมันไม่ยอม loop ให้ผมไม่รุ้ผิดตรงไหน

      <script type="text/javascript">
        function sum() {
          var num_1 = $("#txt_1").val();
          var num_2 = $("#txt_2").val();
          var num_result = "";

          $.ajax({
            type : "POST",
            url : "b.php",
            data : "num_1="+num_1+"&num_2="+num_2,
            cache:false,
            success: function(data) {
              $("#txt_result").val(data); 
            }
          })

        }
      </script>
    </head>

<?php
if(isset($_POST['num_1'])) {
	$num_1 = $_POST['num_1'];
	$num_2 = $_POST['num_2'];

	$num_result = $num_1 * $num_2;

	echo $num_result;	

}
?>

                                     <td width="5%"><center><input name="neworder" type="checkbox" value="<?php echo $objResult["id"];?>"></center></td>
                                    <td width="10%"><center><img src="pics/<?php echo $objResult["picture"];?>" width="80" height="80"></center></td>
                                    <td width="30%"><?php echo $objResult["product_name"];?></td>
                                    <td width="10%"><center><input type="text" name="txt_1" id="txt_1" value="<?php echo $objResult["unitprice"];?>" size="1" onkeyup="JavaScript :sum();" readonly></center></td>
                                    <td width="5%"><center><input type="text" name="txt_2" id="txt_2" size="1" onkeyup="JavaScript :sum();"></center></td>
                                    <td width="5%"><center><input name="addextra" type="checkbox"></center></td>
                                    <td width="5%"><center><input name="addeggtype1" type="checkbox"></center></td>
                                    <td width="5%"><center><input name="addeggtype2" type="checkbox"></center></td>
                                    <td width="15%"><center><input type="text" name="detail"></center></td>
                                    <td width="15%"><center><input type="text" name="txt_result" id="txt_result" size="7" readonly="readonly"></center></td>