写一段MVC模式中模型层的代码

假设我们正在开发一个简单的博客应用程序，以下是一个可能的模型层代码示例：

class PostModel {
    private $db;

    public function __construct($db) {
        $this->db = $db;
    }

    public function getPosts() {
        $query = "SELECT * FROM posts";
        $result = $this->db->query($query);
        return $result->fetchAll();
    }

    public function getPostById($id) {
        $query = "SELECT * FROM posts WHERE id = :id";
        $statement = $this->db->prepare($query);
        $statement->execute(['id' => $id]);
        return $statement->fetch();
    }

    public function createPost($title, $content) {
        $query = "INSERT INTO posts (title, content) VALUES (:title, :content)";
        $statement = $this->db->prepare($query);
        $statement->execute(['title' => $title, 'content' => $content]);
        return $this->db->lastInsertId();
    }

    public function updatePost($id, $title, $content) {
        $query = "UPDATE posts SET title = :title, content = :content WHERE id = :id";
        $statement = $this->db->prepare($query);
        $statement->execute(['id' => $id, 'title' => $title, 'content' => $content]);
        return $statement->rowCount();
    }

    public function deletePost($id) {
        $query = "DELETE FROM posts WHERE id = :id";
        $statement = $this->db->prepare($query);
        $statement->execute(['id' => $id]);
        return $statement->rowCount();
    }
}

这个模型层使用了 PDO 来与数据库进行交互，实现了一些基本的增删改查操作。在一个真正的应用程序中，可能会有更多的方法和复杂的查询逻辑，但这个示例应该可以帮助你理解模型层是如何工作的。