singleton.md

Singleton Design Pattern

The Singleton Pattern ensures that a class has only one instance and provides a global point of access to that instance.

How to apply

class Singleton
{
    private static ?Singleton $instance = null;

    // Prevent object creation
    private function __construct() {}

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

    // Prevent cloning
    private function __clone() {}

    // Prevent serialize
    public function __serialize(): array
    {
        throw new Exception("Cannot serialize a singleton");
    }

    // Prevent unserialize
    public function __unserialize(array $data): void
    {
        throw new Exception("Cannot unserialize a singleton");
    }

    public function printInfo(): void
    {
        echo "Singleton instance";
    }
}

Usage:

$singleton = Singleton::getInstance();
$singleton->printInfo();

Output:

Singleton instance

Note:

__serialize(), __unserialize(), __sleep() and __wakeup() must have public visibility or PHP will generate warning or notice (in previous version).