<?php
class Database
{
    private static $instance = null;

    private function __construct()
    {
        // 데이터베이스 연결 로직
    }

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

    public function query($sql)
    {
        // 쿼리 실행 로직
        echo "Executing query: " . $sql;
    }
}

$db1 = Database::getInstance();
$db1->query("SELECT * FROM users");

$db2 = Database::getInstance();
$db2->query("SELECT * FROM products");

// $db1과 $db2는 동일한 인스턴스입니다.
var_dump($db1 === $db2); // bool(true)
?>

<?php
interface Product
{
    public function getName();
}

class A_Product implements Product
{
    public function getName()
    {
        return "Product A";
    }
}

class B_Product implements Product
{
    public function getName()
    {
        return "Product B";
    }
}

class ProductFactory
{
    public static function createProduct($type)
    {
        switch ($type) {
            case 'A':
                return new A_Product();
            case 'B':
                return new B_Product();
            default:
                throw new Exception("Invalid product type.");
        }
    }
}

$productA = ProductFactory::createProduct('A');
echo $productA->getName(); // Product A

$productB = ProductFactory::createProduct('B');
echo $productB->getName(); // Product B
?>