ajax 三级联动写法

DOCTYPE html>
html lang="en">
head>
    meta charset="UTF-8">
    title>Titletitle>
    script src="../wenjian/jquery-2.2.3.min.js">script>
head>
body>
select id="sheng">
    option>请选择option>
select>
select id="shi">
    option >请选择option>
select>
select id="qu">
    option >请选择option>
select>
body>
html>
script>
    $.ajax({
        data: {parent_id: 0}, //发送的数据
        dataType: "json", //返回值的类型 text为string型
        type: ‘post‘,   //发送请求的方法(get)
        url: ‘sheng_l.php‘,//发送请求的地址
        success: function (data) {//发送成功时的回调函数
    //            console.log(data);
            for (var i in data) {
                $("#sheng").append("
"+ data[i][2] +"‘>" + data[i][1]  +"
")
            }
        }
    })
    $(document).ready(function () {
        $("#sheng").change(function () {
            $("#shi").get(0).options.length= 1;
//            $("#qu").get(0).options.length= 1;
            var data = $("#sheng").find("option:selected").val();
            $.ajax({
                data:{parent_id:data},
                type:"post",
                dataType:"json",
                url:"sheng_l.php",
                success:function (data) {
                    for(var i in data){
                        $("#shi").append("
" + data[i][2] +"‘>" + data[i][1] +"
")
                    }
                }
            })
        })
    })
    $(document).ready(function () {
        $("#shi").change(function () {
            $("#qu").get(0).options.length= 1;
            var data = $("#shi").find("option:selected").val();
            $.ajax({
                data:{parent_id:data},
                type:"post",
                dataType:"json",
                url:"sheng_l.php",
                success:function (data) {
                    for (var i in data){
                        $("#qu").append("
" +data[i][2] +"‘>" +data[i][1] +"
")
                    }
                }
            })
        })
    })

php
/**
 * Created by fcc
 * User: Administrator
 * Date: 2017/10/29
 * Time: 20:56
 */
require_once "../wenjian/DBDA.class.php";
$db = new DBDA();
$sql = "select * from region WHERE father_id = {$_POST[‘parent_id‘]}";
$result = $db->Query($sql);
echo json_encode($result);