What well-known design patterns do you apply when developing system architecture?

When developing system architecture in PHP, the following design patterns are often used:

• Singleton — for creating a single instance of a class, for example, for database connection.
• Factory Method — for creating objects without specifying the exact class, which simplifies extensibility.
• MVC (Model-View-Controller) — separation of application logic, presentation, and control, widely used in web development.
• Dependency Injection — for managing dependencies between classes, improving testability and modularity.
• Observer — for implementing an event-driven model, where objects subscribe to changes in other objects.

Example of Singleton for database connection:

class Database {
    private static $instance = null;
    private function __construct() {
        // connect to DB
    }
    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new Database();
        }
        return self::$instance;
    }
}

The choice of pattern depends on the specific tasks and architectural requirements of the project.