with jQuery

<?php
//........truncated code ..............
$gTotal = 0;
while($menu=mysql_fetch_array($pesanan))
{
$tot=$menu[harga]*$menu[jumlah];
echo "<tr>\n"
    ."<td>$menu[nama_menu]</td>\n"
    ."<td><div align='center'>$menu[jumlah]</td></div>\n"
    ."<td><div align='center'>$menu[harga]</td></div>\n"
    ."<td><div align='center'>$tot</td></div>";
$gTotal = $gTotal + $tot;
}
echo "</tr></table>";
    ."Total Pembelian Anda : Rp. $gTotal<br />\n"
    ."Bayar : Rp. <input name='bayar' type='text' id='bayar' />\n"
    ."<input name='proses' type='submit' id='proses' value='Click' />\n"
    ."<input name='kembali' type='text' id='kembali'/>\n"
    ."<input name='gTotal' type='hidden' id='gTotal' value='$gTotal' />\n" // add this line
    ."</form>";
?>
<script type="text/javascript">
        $('#proses').click(function(){
      var  kembalian;
            {
              gTotal=parseFloat($('#gTotal').val().replace(/[^\d\.\-\ ]/g, ''));;
              bayar=parseFloat($('#bayar').val().replace(/[^\d\.\-\ ]/g, ''));;
              kembalian=gTotal-bayar;
              $('#kembali').val(kembalian);
            }
        });
</script>