php+ajax实现select分类二级菜单效果

<select id="teId" name="te_id" onchange="getArea()"> 
   <option value="1000">--请选择分类--</option> 
      <option value="1">房产知识</option> 
      <option value="8">房屋买卖</option> 
      <option value="16">法律法规</option> 
      <option value="24">银行贷款</option> 
      <option value="31">房产类型</option> 
      <option value="40">装修风水</option> 
      <option value="47">房产经纪</option> 
      <option value="52">其他问题</option> 
      <option value="54">楼盘问答</option> 
     </select> 
  <span id="teTid" ></span> 
  <span></span> 
  <select id="wealth" name="wealth"> 
   <option value="">--请选择悬赏积分--</option> 
   <option value="0">我要给0</option> 
   <option value="5">我要给5</option> 
   <option value="15">我要给15</option> 
   <option value="30">我要给30</option> 
   <option value="50">我要给50</option> 
  </select>

// 创建ajax引擎
function getXmlHttpObject() {
    var xmlHttpRequest;
    if (window.ActiveXObject) {
        xmlHttpRequest = new ActiveXObject("Microsoft.XMLHTTP");
    } else {
        xmlHttpRequest = new XMLHttpRequest();
    }
    return xmlHttpRequest;
}
var myXmlHttpRequest = "";
function getArea() {
    //alert(ID('teId').value);
    myXmlHttpRequest = getXmlHttpObject();
    // 怎样判断是否可以www.111cn.net
    if (myXmlHttpRequest) {
        // post 发送数据
        var url = "ajax_tiwen.php";
        var date = "teId=" + ID('teId').value;
        myXmlHttpRequest.open("post", url, true);
        myXmlHttpRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=utf-8");
        myXmlHttpRequest.onreadystatechange = TiWen;
        myXmlHttpRequest.send(date);
    }
}

<?php
$str = '';
if (!$teamId) {
    $str.= '<select name="te_tid">';
    $str.= ' <option value="1000">--请选择分类--</option>';
    $str.= '</select>';
    echo $str;
    exit;
} else {
    $str.= '<select id="teTid" name="te_tid">';
    $str.= ' <option value="1000">--请选择分类--</option>';
    foreach ($teamId as $v) {
        $str.= '<option value="' . $v['id'] . '">' . $v['tname'] . '</option>';
    }
    $str.= '</select>';
    echo $str;
}
?>