Can you list the main design patterns for classes?

Design patterns for classes in PHP (and in OOP in general) include:

• Singleton — ensures a class has only one instance and provides a global point of access.
• Factory Method — defines an interface for creating an object, allowing subclasses to alter the type of objects created.
• Abstract Factory — provides an interface for creating families of related or dependent objects without specifying their concrete classes.
• Strategy — enables selecting an algorithm's behavior at runtime by encapsulating algorithms in separate classes.
• Decorator — dynamically adds responsibilities to objects by wrapping them.
• Observer — defines a dependency between objects so that when one changes, others are notified.
• Adapter — converts the interface of a class into another interface clients expect.
• Facade — provides a simplified interface to a complex system.

Example of Singleton in PHP:

class Singleton {
    private static $instance = null;

    private function __construct() {}
    private function __clone() {}

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

These patterns help write flexible, extendable, and maintainable code.