PHP でオブジェクト指向設計パターンを使用するにはどうすればよいですか?

interface Shape {
    public function draw();
}

class Circle implements Shape {
    public function draw()
    {
        echo "Drawing a circle";
    }
}

class Square implements Shape {
    public function draw()
    {
        echo "Drawing a square";
    }
}

class ShapeFactory {
    public static function create($type)
    {
        switch ($type) {
            case 'circle':
                return new Circle();
            case 'square':
                return new Square();
            default:
                throw new Exception("Invalid shape type");
        }
    }
}

$circle = ShapeFactory::create('circle');
$circle->draw(); // Output: Drawing a circle

$square = ShapeFactory::create('square');
$square->draw(); // Output: Drawing a square

class Database {
    private static $instance;

    private function __construct()
    {
        // 应该在这里初始化数据库连接
    }

    public static function getInstance()
    {
        if (!self::$instance) {
            self::$instance = new self();
        }
        return self::$instance;
    }
}

$db = Database::getInstance();

class User implements SplSubject {
    private $observers = [];

    public function attach(SplObserver $observer)
    {
        $this->observers[] = $observer;
    }

    public function detach(SplObserver $observer)
    {
        $key = array_search($observer, $this->observers, true);
        if ($key !== false) {
            unset($this->observers[$key]);
        }
    }

    public function notify()
    {
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }
}

class Logger implements SplObserver {
    public function update(SplSubject $subject)
    {
        echo "Logging user update: " . $subject->getName();
    }
}

$user = new User();
$user->attach(new Logger());
$user->setName("John Doe"); // Output: Logging user update: John Doe