给出用PHP和MySQL语言实现论坛（功能包括用户登录和注册，更改密码。发表帖子，回复帖子和删除帖子）代码和表创建代码

表创建代码：

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(50) NOT NULL,
  `password` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `post` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(255) NOT NULL,
  `content` text NOT NULL,
  `author_id` int(11) NOT NULL,
  `created_at` datetime NOT NULL,
  PRIMARY KEY (`id`),
  KEY `author_id` (`author_id`),
  CONSTRAINT `post_ibfk_1` FOREIGN KEY (`author_id`) REFERENCES `user` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `comment` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `content` text NOT NULL,
  `author_id` int(11) NOT NULL,
  `post_id` int(11) NOT NULL,
  `created_at` datetime NOT NULL,
  PRIMARY KEY (`id`),
  KEY `author_id` (`author_id`),
  KEY `post_id` (`post_id`),
  CONSTRAINT `comment_ibfk_1` FOREIGN KEY (`author_id`) REFERENCES `user` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `comment_ibfk_2` FOREIGN KEY (`post_id`) REFERENCES `post` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

PHP代码：

<?php
// 连接数据库
$conn = mysqli_connect('localhost', 'root', 'password', 'forum');

// 检查连接是否成功
if (!$conn) {
    die('连接数据库失败: ' . mysqli_connect_error());
}

// 处理注册请求
if (isset($_POST['register'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];

    // 检查用户名是否已经存在
    $check_sql = "SELECT * FROM `user` WHERE `username` = '$username'";
    $result = mysqli_query($conn, $check_sql);

    if (mysqli_num_rows($result) > 0) {
        echo '<script>alert("用户名已经存在");</script>';
    } else {
        // 插入新用户记录
        $insert_sql = "INSERT INTO `user` (`username`, `password`) VALUES ('$username', '$password')";
        if (mysqli_query($conn, $insert_sql)) {
            echo '<script>alert("注册成功");</script>';
        } else {
            echo '<script>alert("注册失败，请稍后重试");</script>';
        }
    }
}

// 处理登录请求
if (isset($_POST['login'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];

    // 检查用户名和密码是否匹配
    $check_sql = "SELECT * FROM `user` WHERE `username` = '$username' AND `password` = '$password'";
    $result = mysqli_query($conn, $check_sql);

    if (mysqli_num_rows($result) > 0) {
        // 登录成功，记录用户信息到 session 中
        session_start();
        $_SESSION['username'] = $username;
        $_SESSION['user_id'] = mysqli_fetch_assoc($result)['id'];

        header('Location: index.php');
        exit();
    } else {
        echo '<script>alert("用户名或密码错误");</script>';
    }
}

// 处理发帖请求
if (isset($_POST['create_post'])) {
    $title = $_POST['title'];
    $content = $_POST['content'];
    $author_id = $_SESSION['user_id'];
    $created_at = date('Y-m-d H:i:s');

    // 插入新帖子记录
    $insert_sql = "INSERT INTO `post` (`title`, `content`, `author_id`, `created_at`) VALUES ('$title', '$content', $author_id, '$created_at')";
    if (mysqli_query($conn, $insert_sql)) {
        echo '<script>alert("发帖成功");</script>';
    } else {
        echo '<script>alert("发帖失败，请稍后重试");</script>';
    }
}

// 处理回帖请求
if (isset($_POST['create_comment'])) {
    $content = $_POST['content'];
    $author_id = $_SESSION['user_id'];
    $post_id = $_POST['post_id'];
    $created_at = date('Y-m-d H:i:s');

    // 插入新评论记录
    $insert_sql = "INSERT INTO `comment` (`content`, `author_id`, `post_id`, `created_at`) VALUES ('$content', $author_id, $post_id, '$created_at')";
    if (mysqli_query($conn, $insert_sql)) {
        echo '<script>alert("回帖成功");</script>';
    } else {
        echo '<script>alert("回帖失败，请稍后重试");</script>';
    }
}

// 处理删除帖子请求
if (isset($_POST['delete_post'])) {
    $post_id = $_POST['post_id'];

    // 删除帖子及其相关评论
    $delete_post_sql = "DELETE FROM `post` WHERE `id` = $post_id";
    $delete_comment_sql = "DELETE FROM `comment` WHERE `post_id` = $post_id";
    if (mysqli_query($conn, $delete_post_sql) && mysqli_query($conn, $delete_comment_sql)) {
        echo '<script>alert("删除帖子成功");</script>';
    } else {
        echo '<script>alert("删除帖子失败，请稍后重试");</script>';
    }
}

注意：以上代码仅供参考，需要根据具体情况进行修改和完善。