如何更新latlng从javascript到php在谷歌地图v3

我必须在地图被拖动后在地图上显示一堆标记。我如何通过当前地图中心从javascript到我的php代码来获取数据?latlng没有更新。

var CHANGE_MARKERS = false;
var CITY_MAP_CENTER_LAT= '12.971598';
var CITY_MAP_CENTER_LNG= '77.594569';
var CITY_MAP_ZOOMING_FACT= 15;
var CITY_MAP_VIEW_TYPE = '';
var LAT = "";
var LNG = "";
var LATLNG = "";
var A1,A2,A3,A4;
var map;
var mgr;
var mc = null;
var markerClusterer = null;
var showMarketManager = false;
var infoWindow;
var markersarray = [];
var allmarkers = [];
function setLatLng(){
    if(CHANGE_MARKERS == false){
        LAT = '12.971598';
        LNG = '77.594569';
    }
    else{
        var ctr = map.getCenter();
        LAT =ctr.lat();
        LNG =ctr.lng();
    }
}
function setMarkers(){
    allmarkers.length = 0;
    allmarkers = [];
    var markers = [];
    if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp=new XMLHttpRequest();
    }
    else
    {// code for IE6, IE5
    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange=function()
    {
    if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
       alert(<?php echo $_SESSION['center']; ?>);
       //var markersData = xmlhttp.responseText;
       //alert(markersData);
       markers = {'data':[
           <?php

           ?>
       ]};
       for (var level in markers) {
            //alert(markers[level].length);
       }
    }
    }
    xmlhttp.open("GET","../subscriber_listing/google.php?center=" + map.getCenter(),false);
    xmlhttp.send();
    if (markers) {
         for (var level in markers) {
            //alert(markers[level].length);
            for (var i = 0; i < markers[level].length; i++) {
               var details = markers[level][i];
               //alert(details.name);
               var image = new google.maps.MarkerImage(details.icons,new google.maps.Size(PIN_POINT_ICON_WIDTH, PIN_POINT_ICON_HEIGHT));
               var myLatLng = new google.maps.LatLng(details.location[0], details.location[1]);
               var newmarker = createMarker(details.name,myLatLng,image);
               allmarkers.push(newmarker);
            }
            mgr.addMarkers( allmarkers, 0 );
         }
         mgr.refresh();
      }
}
function doSearch(){
    display();
    CHANGE_MARKERS = true;
    initialize();
}
function createMarker(name,latlng,image){
    var marker = new google.maps.Marker({
          title: name,
          position: latlng,
          icon: image,
          clickable: true,
          draggable: false,
          flat: true
   });
   return marker;
}
function display(){
    if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp=new XMLHttpRequest();
    }
    else
    {// code for IE6, IE5
    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange=function()
    {
    if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    }
    }
    xmlhttp.open("GET","../subscriber_listing/google.php?center=" + map.getCenter(),false);
    xmlhttp.send();
}
if(CITY_MAP_CENTER_LAT=='')
{
    var CITY_MAP_CENTER_LAT = 34;
}
if(CITY_MAP_CENTER_LNG=='')
{
    var CITY_MAP_CENTER_LNG = 0;
}
if(CITY_MAP_CENTER_LAT!='' && CITY_MAP_CENTER_LNG!='' && CITY_MAP_ZOOMING_FACT!='')
{
    var CITY_MAP_ZOOMING_FACT = 13;
}else if(CITY_MAP_ZOOMING_FACT!='')
{
    var CITY_MAP_ZOOMING_FACT = 3;
}
var PIN_POINT_ICON_HEIGHT = 32;
var PIN_POINT_ICON_WIDTH = 20;
if(CITY_MAP_VIEW_TYPE=='TERRAIN')
{
    var maptype= google.maps.MapTypeId.TERRAIN;
}else
if(CITY_MAP_VIEW_TYPE=='SATELLITE')
{
    var maptype= google.maps.MapTypeId.SATELLITE;
}else
if(CITY_MAP_VIEW_TYPE=='')
{
    var maptype= google.maps.MapTypeId.ROADMAP;
}
if(MAP_DISABLE_SCROLL_WHEEL_FLAG)
{
    var MAP_DISABLE_SCROLL_WHEEL_FLAG = 'No';
}

function initialize() {
   setLatLng();
   var myOptions = {
        zoom: CITY_MAP_ZOOMING_FACT,
        center: new google.maps.LatLng(LAT, LNG),
        mapTypeId: maptype
    }
    map = new google.maps.Map(document.getElementById("map_canvas"),myOptions);
    mgr = new MarkerManager(map);
    infoWindow = infoWindow = new google.maps.InfoWindow();
   google.maps.event.addListener(mgr, 'loaded', function(){
         setMarkers();
   });

    google.maps.event.addListener(map, "dragend", function() {
            doSearch();
    });
}
google.maps.event.addDomListener(window, 'load', initialize);