ext异步加载树的子节点

<script>
           Ext.onReady(function(){
            var root=new Ext.tree.AsyncTreeNode({
               id:'0',
               text:'所有分类'
            });
            var data=new Ext.tree.TreeLoader({url:'_tree_json.php'});
            var tree=new Ext.tree.TreePanel({
               renderTo:'test',
               root:root,
               loader:data,
               width:300
            });
            data.on('beforeload',function(treeLoader,node){
               this.baseParams.id=node.attributes.id;
            },data);
         });
       
        </script>
      <div id="test"></div>
     
      服务端的php代码：

<?php
require("conn.php");
$father_id=$_POST['id'];
function _count($father_id){
   $count_req=mysql_query("select count(id) as count from kind where father_id=$father_id");
   $count_num=mysql_fetch_array($count_req);
   return $count_num['count'];
}
$count=_count($father_id);
$return=mysql_query("select * from kind where father_id=$father_id");
echo "[";
$i=1;
while ($req=mysql_fetch_array($return)) {
   $id=$req['id'];
   $name=$req['name'];  
   if (_count($id)>0) {
        echo "{
       id: $id,
       text: '$name',
       leaf: false
     }";     
   }else {
      echo "{
       id: $id,
       text: '$name',
       leaf: true
     }";
   }  
   if ($i<$count) {
      echo ",";
   }  
   $i++;
}
echo "]";

?>