datatable无法发送ajax

 JS脚本如下：
<script type="text/javascript">
//<![CDATA[
  $(document).ready(function(){
    $('#tpval').focus();
    var s = document.getElementById('num');
    if (typeof(s) != "undefined") {
      $('#num').focus();
    }


            $.extend( true, $.fn.dataTable.defaults, {
                "bFilter": false,
                } );
            $('#datastable').dataTable({
                "bStateSave": true,
                "bProcessing": true,
                "bServerSide": true,
              "sAjaxSource": "/webadmin/idxg.php?gmd=pay.lists&ajax=1",
                "sServerMethod": "POST",
                "bJQueryUI": true,
                "sPaginationType": "full_numbers",
                "fnRowCallback": function( nRow, aData, iDisplayIndex ) {
                    /* Append the grade to the default row class name */   
                    switch ( aData[11] ) {
                        case "1": /* 退单 */
                            $("td", nRow).each(function (){$(this).wrapInner("<font style='color:#ff0000;'><\/font>");});
                            break;
                        case "2": /* 测试单 */
                            $("td", nRow).each(function (){$(this).wrapInner("<font style='color:#ff9000;'><\/font>");});
                            break;
                    }
                    return nRow;
                },
                "aoColumnDefs": [
                  { "bSortable": false, "aTargets": [ 0 ] }
                ],
                "aaSorting": [[ 1, 'asc' ]]
            });
          });

//]]>
</script>
服务端：
public function lists()
    {
        $pri = self::_check_pri(array(44,59,66));
        **$ajax = getgpc('ajax','G');**
        //接收待查询、显示的数据库字段
        $select_keys = array(' ', 'pay_id','transaction_type','transaction_id','receiver_id','pay_type','pay_money','pay_date','order_state','order_money','order_id','order_date','check_date','buyer_id');
        // ajax请求
        if ($ajax) {
            if (!is_array($pri) || (!$pri[44] && !$pri[66])) {
                exit;
            }
            $pay_hide = 0;
            if ($pri[59]) {
                $pay_hide = 1;
            }
            self::_lists_ajax($select_keys, $pay_hide);
        }

                现象：在服务端打印$ajax为NULL，不懂得该如何调试从前端到服务端过程中是否有ajax通信

你的getgpc怎么写的，你参数放url里面是get提交，要用$_GET，不是$_POST

 $ajax =$_GET["ajax"];//直接这样呢？