如何像whatsapp一样使用php，ajax在聊天中排列消息

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

$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 接收新消息
if (isset($_POST['message'])) {
    $message = $_POST['message'];
    
    // 将消息插入数据库
    $sql = "INSERT INTO messages (message) VALUES ('$message')";
    $conn->query($sql);
}

// 获取聊天消息
$sql = "SELECT * FROM messages ORDER BY id DESC";
$result = $conn->query($sql);

// 输出聊天消息
if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        echo "<p>" . $row['message'] . "</p>";
    }
} else {
    echo "暂无消息";
}

$conn->close();
?>

// 发送消息
function sendMessage() {
    var message = document.getElementById("message").value;
    var xmlhttp = new XMLHttpRequest();
    
    xmlhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            // 刷新消息列表
            document.getElementById("message-list").innerHTML = this.responseText;
        }
    };
    
    xmlhttp.open("POST", "backend.php", true);
    xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xmlhttp.send("message=" + message);
}

// 定时刷新消息列表
setInterval(function() {
    var xmlhttp = new XMLHttpRequest();
    
    xmlhttp.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            // 刷新消息列表
            document.getElementById("message-list").innerHTML = this.responseText;
        }
    };
    
    xmlhttp.open("GET", "backend.php", true);
    xmlhttp.send();
}, 1000);