codice:
Class Counter {
        static private $num = 0;
        static public function increment() {
                Counter::$num++;
        }
        static public function decrement() {
                Counter::$num--;
        }
        static public function get() {
                return Counter::$num;
        }
}

// + 3
Counter::increment();
Counter::increment();
Counter::increment();

// - 1
Counter::decrement();


echo Counter::get(); // = 2