如何使用ajax刷新评论？

var xmlhttp;
if (window.XMLHttpRequest) {
   // 针对大多数浏览器
   xmlhttp = new XMLHttpRequest();
} else {
   // 针对IE6, IE5
   xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}

// 定义回调函数
xmlhttp.onreadystatechange = function() {
   if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
       // 处理从服务器返回的数据
       var response = xmlhttp.responseText;
       // 更新评论区域的内容
       document.getElementById("comment-area").innerHTML = response;
   }
};

// 获取评论区域的元素
var commentArea = document.getElementById("comment-area");

// 定义刷新评论的事件处理函数
function refreshComments() {
   // 发送Ajax请求到服务器
   xmlhttp.open("GET", "refresh_comments.php", true);
   xmlhttp.send();
}

// 在适当的时机调用刷新评论的函数
commentArea.addEventListener("click", refreshComments);

<?php
// 连接数据库
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
   die("连接数据库失败: " . $conn->connect_error);
}

// 查询最新的评论数据
$sql = "SELECT * FROM comments ORDER BY created_at DESC";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
   // 构建评论列表的HTML内容
   $commentsHtml = "";
   while($row = $result->fetch_assoc()) {
       $commentsHtml .= "<div class='comment'>";
       $commentsHtml .= "<p class='author'>" . $row["author"] . "</p>";
       $commentsHtml .= "<p class='content'>" . $row["content"] . "</p>";
       $commentsHtml .= "<p class='timestamp'>" . $row["created_at"] . "</p>";
       $commentsHtml .= "</div>";
   }
   
   // 将评论列表返回给前端
   echo $commentsHtml;
} else {
   echo "暂无评论";
}

$conn->close();
?>