写个Ajax三级联动实例，用的最新的全国省市县mysql数据。这里只说一下思路，具体请看源码。
首先看index.php，这是一个省份列表
<?php
  $conn=mysql_connect("localhost","root","");
  mysql_select_db("china");
  mysql_query("set names 'utf8'");
  $sql="select * from t_province order by ProSort ASC";
  $result=mysql_query($sql);
  echo "<from id='form1'>\n";
  echo "<select id='province' οnchange='queryCity(this.options[this.selectedIndex].value)'>\n";
  echo "<option value='-1' selected>请选择省份</option>\n";
  while($row=mysql_fetch_row($result)){
    echo "<option value='$row[0]'>$row[1]</option>\n";
  }
  echo "</select>\n";
  echo "<span id='city'></span>\n";
  echo "<span id='area'></span>\n";
  echo "</form>\n";
?>
还有两个javascript代码作用是传递省份主键值和城市主键值到ajax.php中查询数据。
function queryCity(province){
  createXMLHttpRequest();
  type="city";
  var url="ajax.php?type=province&province="+province;
  xmlHttp.open("GET",url,true);
  xmlHttp.onreadystatechange=handleStateChange;
  xmlHttp.send(null);
}

function queryArea(city){
  createXMLHttpRequest();
  type="area";
  var url="ajax.php?type=city&city="+city;
  xmlHttp.open("GET",url,true);
  xmlHttp.onreadystatechange=handleStateChange;
  xmlHttp.send(null);
}
ajax.php文件就是接受传递过来的数据并查询相关市县。
// 根据传递的省份ID值查询t_city表中ProID值为该省份ID的城市。
if($_GET['type']=='province'){
  if(!empty($_GET['province'])){
                                $province = intval($_GET['province']);
    $sql="select * from `t_city` where `ProID` = '$province' ";
    $result=mysql_query($sql);
    echo "<select οnchange='queryArea(this.options[this.selectedIndex].value)'>\n";
    echo "<option value='-1' selected>请选择城市</option>\n";
    while($row=mysql_fetch_row($result)){
      echo "<option value='$row[0]'>$row[1]</option>\n";
    }
    echo "</select>\n";
  }
}
// 根据传递的城市ID值查询t_district表中CityID值为该市ID的城市。
if($_GET['type']=='city'){
  if(!empty($_GET['city'])){
                                $city = intval($_GET['city']);
    $sql="select * from `t_district` where `CityID` = '$city'";
    $result=mysql_query($sql);
    echo "<select>\n";
    echo "<option value='-1' selected>请选择县</option>\n";
    while($row=mysql_fetch_row($result)){
      echo "<option value='$row[0]'>$row[1]</option>\n";
    }
    echo "</select>\n";
  }
}