Javascript动态数学

Question

I am trying to have JavaScript do math on a forum field when the user clicks out of the field, and then display their total in a div right beside the field they filled out. Here is my code:

 <script type="text/javascript">

 function Calc(){
 VField1=eval(document.calcform.sidewalk_days_close.value)
 VField2=eval(document.calcform.<?php print $SNR;?>.value)
 d=VField1*VField2
 document.row1.field4output.value = d;
 }

 </script>

And my HTML & PHP:

 $SNR = 1;      // Sidewalks and non-roadway areas Value to Multiply

 <td align="center"><p><label><span class="small">Amount of Days Closed</span><br />
     <input id="sidewalk_days_close" type="text" name="sidewalk_days_close" size="10"      maxlength="10" value="
 <?php if (isset($_POST['sidewalk_days_close'])) {print      htmlspecialchars($_POST['sidewalk_days_close']); } ?>" onchange="javascript:Calc();"/>
 <?php if (isset($_POST['sidewalk_days_close'])) {
            $SDC = trim($_POST['sidewalk_days_close']);
            $SDC = strip_tags($SDC);
    }?>


   </label></p><hr /></td>
     <td align="center"><div id="row1"></div></td>