button of change quantity problem

<?php
  error_reporting(0);
  session_start();
  header('Content-type: text/html; charset=utf-8');
  include_once('../../xajax_core/xajax.inc.php');  // ?? xajax
  $xajax = new xajax();            // ?? xajax ??

  // ???????
  $chaObj=$xajax->registerFunction('change_quantity');
  $chaObj->useSingleQuote();
  $chaObj->addParameter(XAJAX_FORM_VALUES,'form1');
  $xajax->processRequest();        // ????

  $xajax->printJavascript();


  //---------------------- ????? ---------------------
  // ?????????????

  function change_quantity($form) {

    $objResponse = new xajaxResponse();  // ??????
    // ???? Checkbox ??? p_id, ????, ??????
    // ?????????? 'p_id' ??, ??????????
    // ????????

    if(isset($form['p_id'])) {
      // ?????????????
      foreach($form['p_id'] as $p_id) {
        // ???????????????
        if($form['qua'][$p_id]=='0') {
          // ???????? 0, ????????????
          unset($_SESSION['cart'][$p_id]);

          if(count($_SESSION['cart'])==0) {  // ????????
            $objResponse->                   // ?????
              script('alert("cart has no items");window.close();');
              unset($_SESSION['cart']);
          }
          else  { // ???????????
            // ?????????????
            $objResponse->remove($p_id);

            // ???????????
            $objResponse->assign('total','innerHTML',gettotal());
          }
        }
        else if ($form['qua'][$p_id]>0) {
          // ????? 0 ??, ?????????????
          $_SESSION['cart'][$p_id]['quantity'] = $form['qua'][$p_id];

          // ??????, ?????????????????
          $objResponse->assign('sub[' . $p_id . ']', 'innerHTML',
                               $_SESSION['cart'][$p_id]['quantity'] *
                               $_SESSION['cart'][$p_id]['price']);
          $objResponse->assign('total','innerHTML',gettotal());
        }
        else {
          // ??????, ?????????
          $objResponse->assign('qua[' . $pid . ']', 'innerHTML',
                               $_SESSION[$pid]['quantity']);
        }
      }
    }
    return $objResponse;          // ??????
  } // ???? change_quantity() ??

  // ????????
  function getTotal() {
    $total = 0;
    foreach($_SESSION['cart'] as $p_id => $item)
      $total += ($item['quantity'] * $item['price']);

    return $total;
  }


  //---------------------- ??? HTML  ---------------------
  // ?????????, ?????????????????
  if( !isset($_SESSION['cart']) ) { // ???????
    echo "<script>alert(\"you haven't chosen any products\");" .
         "window.close();</script>";
    exit();
  }
?>
<title>?????</title>


<!-- -------------------- ??? JavaScript ------------------ -->
<script type="text/javascript">
// ?????????????? checkbox ???
function select_all(formName, elementName, selectAllName) {
  elem = document.forms[formName].elements[elementName];
  if(!elem) // ??????
    return;
  else if(elem.length!= null) // ????????? (elem ???)
    for(var i = 0; i < elem.length; i++)
      elem[i].checked =
        document.forms[formName].elements[selectAllName].checked;
  else
    elem.checked =
        document.forms[formName].elements[selectAllName].checked;
}

// ?????????????0, ????????
function settozero(formName, elementName) {

  elem = document.forms[formName].elements[elementName];
  if(!elem) // ??????
    return;
  else if(elem.length!= null) { // ????????? (elem ???)
    for(var i = 0; i < elem.length; i++)
      if(elem[i].checked) {
        var qua =
            document.getElementById("qua[" + elem[i].value + "]");
        var subtotal = document.getElementById("sub[" + elem[i].value + "]");


        qua.value = 0;  // ????? 0
        subtotal.value = 0;


      }
  }
  else {
    var qua = document.getElementById("qua[" + elem.value + "]");
    var subtotal = document.getElementById("sub[" + elem.value + "]");


    qua.value = 0;  // ????? 0
    subtotal.value = 0;

  }
  var total_value = document.getElementById("total");
  total_value.value = 0;
  <?php $chaObj->printScript(); // ???????????? ?>
}
</script>
<?php $xajax->printJavaScript('/'); ?>


<!-- ------------------- ???????? -------------------  -->
<link rel="StyleSheet" type="text/css" href="../module.css" />
<form name="form1" id="form1" method="post" action="checkout.php">
<table width="800" border="0" align="center"
       cellspacing="0" cellpadding="2"
       style="text-align:center;border:1px solid silver">
  <tr><th colspan="7">cart</th></tr>
  <tr style="background-color:silver;color:white">
    <td height="23" width="80">all
        <!----- ???????????? checkbox ----->
        <input type="checkbox" name="all"
               onClick="select_all('form1','p_id[]',this.name);">
    </td>
    <td height="23" width="460">product name</td>
    <td height="23" width="60">unit price</td>
    <td width="10" height="23"></td>
    <td height="23" width="60">quantity</td>
    <td width="10" height="23"></td>
    <td width="70" height="23">subtotal</td>
  </tr>


<!-- -------------------- ??????? --------------------  -->
<?php
  // ?????????
  $total = 0;
  foreach($_SESSION['cart'] as $p_id => $item) {
    // ???? id ???? $p_id, ???????
    echo "<tr id='$p_id'>\n";
    echo "<td width='80' height='21'>" .
         "<input type='checkbox' name='p_id[]' value='" .
         $p_id . "'></td>\n";
    echo "<td>" . $item['name'] . "</td>\n";
    echo "<td width='60' >" . $item['price'] . "</td>\n";
    echo "<td width='10' style='border-width:0'>×</td>\n";
    echo "<td width='60'>" .
         "<input type='text' name='qua[$p_id]' id='qua[$p_id]'
         value='" . $item['quantity'] . "' size='3'></td>";
    echo "<td width='10' style='border-width:0'>=</td>\n";
    echo "<td width='70' align='right'>" ."<input type='text' size='6' id='sub[$p_id]' value='".($item['quantity'] * $item['price'])."' disabled>"."</td></tr>\n";
  }
?>
  <tr>
    <td align="right" colspan="6">total:</td>
    <td align="right" style="border-top:1px solid silver;width:70">
        <input type="text" name="total" value="<?php echo gettotal(); // display total amount ?>" id="total">
    </td>
  </tr>
</table>


<!-- -------------------- cart button --------------------  -->
<table width="800" border="0" align="center">
  <tr>
    <td height="23" align="center">
        <input type="button" name="DEL" value="remove products"
               onClick="settozero('form1','p_id[]')">
    </td>
    <td height="23" align="center">
      <input type="button" name="UPD" value="change quantity"
             onClick="xajax_change_quantity(xajax.getFormValues('form1'))">
    </td>
    <td height="23" align="right">
        <input type="button" name="CONT" value="continue shopping"
               onClick="window.close();">
    </td>
    <td height="23" align="right">
        <input type="button" name="CONT" value="checkout"
               onClick="location.href='checkout.php';">
    </td>
  </tr>
</table>
</form>