在ajax中使用会话

Question

Im trying to use session in my file called response.php,but I face some problem to do it.

This is table.php(pass some data to response.php):

function ajaxAction(action) 
        {   
            data = $("#frm_"+action).serializeArray();
            $.ajax({
              type: "POST",  
              url: "response.php",  
              data: data ,
              dataType: "json",       
              success: function(response)  
              {
                $('#'+action+'_model').modal('hide');
                $("#employee_grid").bootgrid('reload');
              }   
            });
        } 

response.php:

<?php

include_once("connection.php");
 session_start();
$db = new dbObj();
$connString =  $db->getConnstring();
$params = $_REQUEST;

    function getRecords($params) {
    // getting total number records without any search
    $tid = $_SESSION['tmid'];
    $sql = "SELECT * FROM `cusinfo` where AGENT_CODE_STAFF_ID = $tid ";
    $sqlTot .= $sql;
    $sqlRec .= $sql;

    //concatenate search sql if value exist
    if(isset($where) && $where != '') 
    {
        $sqlTot .= $where;
        $sqlRec .= $where;
    }
    if ($rp!=-1)
    $sqlRec .= " LIMIT ". $start_from .",".$rp;

    $qtot = mysqli_query($this->conn, $sqlTot) or die("error to fetch tot customer data");
    $queryRecords = mysqli_query($this->conn, $sqlRec) or die("error to fetch customer data");

    while( $row = mysqli_fetch_assoc($queryRecords) ) 
    { 
        $data[] = $row;
    }
$json_data = array(
        "current"       => intval($params['current']), 
        "rowCount"      => 10,          
        "total"         => intval($qtot->num_rows),
        "rows"          => $data   // total data array
        );

    return $json_data;
  }
?>

Now the session no working,may I know which code should I change?And how to use session in ajax.Thank for advanced