sum function

<?php
$con = mysql_connect("localhost","root","kathal");
mysql_select_db("arrow", $con);
$result = mysql_query("SELECT sum(itemqty) FROM purbill ");
if ($result)
$query_data=mysql_fetch_array($result);
$ptotal=$query_data["sum(itemqty)"];
$result = mysql_query("SELECT sum(saleqty) FROM salebill ");
if ($result)
$query_data=mysql_fetch_array($result);
$stotal=$query_data["sum(saleqty)"];
$total=$ptotal-$stotal;
echo $total;

?>