PHP+ajax实现二级联动【post+json方式】

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>PHP+ajax实现二级联动</title>
</head>
<body>
    <select id="province">
        <option value="0">请选择省份</option>
        <option value="1">北京</option>
        <option value="2">河北</option>
        <option value="3">山东</option>
    </select>
    <select id="city">
        <option value="0">请选择城市</option>
    </select>
</body>
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script>
$(function(){
    $("#province").change(function(){

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