Ajax调用并维护类结构

Hello I am wondering what is the common method of structuring ajax and php? I am doing a website/application which have a lot of ajax calls. Also I do have a class in php I use for my main php code for keeping track of the user.

When I do ajax calls with jquery to smaller php files I need to re-declare my object I have in other files. I want the object that I have in my "main" php file. What is a good method to get the same object? Right now I am using jquery to pass the values from a div element (which comes from the object previously).

I feel this is not the best practice. I want to do it structured.

Example: I store the user session in User. I have a button a user can click to get some information about his data. For this I have a jquery ajax call which sends and retrieves data for display. In that php file I do have to redo everything again, even if I include the class file, because the object is declared elsewhere.

Example. User can create lists in this kind of way.

I have the main index file which includes this file of new User. This type of stuff.

$user = new User();

if($_SESSION["user_connected"])
{
  $user_profile = $twitter->getUserProfile();
  $displayname = $user_profile->displayName;
}
$user->username = $displayname;
// Also user has an array of lists

jQuery

$('#createlistform').submit(function(event)
{
  var newlistname = $('#listname').val();
  var createlistRequest = $.ajax(
      {
        url: "ajax/ajax_add_list.php",
        dataType: "html",
        type: "POST",
        data: {
          listname: newlistname
        }
      }
  );

the ajax_add_list file ajax is calling below. Now what I do to get this work is sending the username through as a data parameter to the php file from picking it up by $('#username').text() then set a new user with that username. It is not in the example now... but you get the point I think.

include_once("../user.class.php");
$theUser = new User(); 
if(class_exists('User'))
{
echo "yes class exists";
  $newlistname = $_POST['listname'];
  $theUser->createList($newlistname);
}

I wish I could get the same User object that was defined in my index file. I don't wanna create a new one. Although if this is how people do it when they have ajax I'll do it. I just want to know how you usually go by when dealing with different files here and there with javascript getting data from various places, while still maintaining a good structure with class and object.