Comment utiliser PHP pour développer un système de forum simple

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

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

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

<?php
// 注册用户
if(isset($_POST['register'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];
    $email = $_POST['email'];

    // 将用户信息插入到数据库中
    $sql = "INSERT INTO users (username, password, email) VALUES ('$username', '$password', '$email')";
    $conn->query($sql);
}

// 用户登录
if(isset($_POST['login'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];

    // 检查用户名和密码是否匹配
    $sql = "SELECT * FROM users WHERE username='$username' AND password='$password'";
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        // 登录成功
    } else {
        // 登录失败
    }
}
?>

<?php
// 显示帖子列表
$sql = "SELECT * FROM posts";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        echo "标题: " . $row["title"]. " - 作者: " . $row["author"]. "<br>";
    }
} else {
    echo "没有帖子";
}

// 发布帖子
if(isset($_POST['publish'])) {
    $title = $_POST['title'];
    $content = $_POST['content'];
    $author = $_SESSION['username'];

    // 将帖子信息插入到数据库中
    $sql = "INSERT INTO posts (title, content, author) VALUES ('$title', '$content', '$author')";
    $conn->query($sql);
}
?>

<?php
// 显示帖子内容和回复
$post_id = $_GET['post_id'];
$sql = "SELECT * FROM posts WHERE id=$post_id";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    $row = $result->fetch_assoc();
    echo "标题: " . $row["title"]. "<br>";
    echo "内容: " . $row["content"]. "<br>";

    // 显示回复
    $sql = "SELECT * FROM comments WHERE post_id=$post_id";
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        while($row = $result->fetch_assoc()) {
            echo "回复: " . $row["content"]. " - 作者: " . $row["author"]. "<br>";
        }
    } else {
        echo "没有回复";
    }
} else {
    echo "帖子不存在";
}

// 回复帖子
if(isset($_POST['reply'])) {
    $content = $_POST['content'];
    $author = $_SESSION['username'];

    // 将回复信息插入到数据库中
    $sql = "INSERT INTO comments (content, author, post_id) VALUES ('$content', '$author', $post_id)";
    $conn->query($sql);
}
?>