如何使用PHP开发点餐系统的在线客服功能？

<?php
// 连接数据库
$conn = Mysqli_connect("localhost", "username", "password", "database");

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    // 获取客户ID和问题
    $customer_id = $_POST["customer_id"];
    $question = $_POST["question"];

    // 将问题保存到数据库
    $sql = "INSERT INTO customer_service (customer_id, question) VALUES ('$customer_id', '$question')";
    mysqli_query($conn, $sql);
}

if ($_SERVER["REQUEST_METHOD"] == "GET") {
    // 获取客户ID和回复
    $customer_id = $_GET["customer_id"];
    $answer = $_GET["answer"];

    // 更新数据库中对应客户的回复
    $sql = "UPDATE customer_service SET answer='$answer' WHERE customer_id='$customer_id'";
    mysqli_query($conn, $sql);
}

// 关闭数据库连接
mysqli_close($conn);
?>

setInterval(function() {
    // 获取客户ID和最后一条回复的时间
    var customer_id = "123";
    var last_reply_time = "2022-01-01 00:00:00";

    // 发送请求到后台获取回复
    var xhr = new XMLHttpRequest();
    xhr.open("GET", "customer_service.php?customer_id=" + customer_id + "&last_reply_time=" + last_reply_time, true);
    xhr.onreadystatechange = function() {
        if (xhr.readyState == 4 && xhr.status == 200) {
            // 解析回复并在界面上显示
            var reply = JSON.parse(xhr.responseText);
            document.getElementById("reply").innerHTML = reply.answer;
        }
    }
    xhr.send();
}, 5000); // 每5秒获取一次回复