html>
head>
title>www.jb51.net 二级联动/title>
meta http-equiv="Content-Type" content="text/html;charset=utf8">
script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js">/script>
style>
#area-box{width:500px;height:400px;margin:0 auto;border:0px solid #dddddd;text-align:center;}
.area-select{width:200px;height:30px;margin-top:30px;margin-left:30px;}
/style>
head>
body>
  div id="area-box">
    select class="area-select" id='address'>
      option value="0">请选择省份/option>
      option value="1">四川/option>
      option value="2">河北/option>
      option value="3">湖南/option>
    /select>
    select class="area-select" id="city">
      option>请选择城市/option>
    /select>
  /div>
  script>
  $(function(){
    //初始化数据
    var url = 'address.php'; //后台地址
    $("#address").change(function(){ //监听下拉列表的change事件
      var address = $(this).val(); //获取下拉列表选中的值
      //发送一个post请求
      $.ajax({
        type:'post',
        url:url,
        data:{key:address},
        dataType:'json',
        success:function(data){ //请求成功回调函数
          var status = data.status; //获取返回值
          var address = data.data;
          if(status == 200){ //判断状态码，200为成功
            var option = '';
            for(var i=0;iaddress.length;i++){ //循环获取返回值，并组装成html代码
              option +='option>'+address[i]+'/option>';
            }
          }else{
            var option = 'option>请选择城市/option>'; //默认值
          }
          $("#city").html(option); //js刷新第二个下拉框的值
        },
      });
    });
  });
  /script>
/body>

?php
  $key = $_POST['key']; //获取值
  $address[1] = array('成都','绵阳','德阳');
  $address[2] = array('石家庄','唐山','秦皇岛');
  $address[3] = array('长沙','株洲','湘潭');
  if(!empty($address[$key])){ //有值，组装数据
    $result['status'] = 200;
    $result['data'] = $address[$key];
  }else{ //无值，返回状态码220
    $result['status'] = 220;
  }
  echo json_encode($result); //返回JSON数据
?>