Singleton exercise

singleton-exercise.php

<?php

class SingletonClass
{
    private static ?SingletonClass $instance = null;
    private int $counter;

    private function __construct()
    {
        $this->counter = 0;
    }

    public static function GetInstance() : SingletonClass
    {
        if(!self::$instance)
        {
            self::$instance = new SingletonClass();
        }

        return self::$instance;
    }

    public function PrintAndIncrease()
    {
        $this->counter++;
        echo "Counter: ".$this->counter."<br/>";
    }
}

$instance = SingletonClass::GetInstance();
$instance->PrintAndIncrease();

$instance2 = SingletonClass::GetInstance();
$instance2->PrintAndIncrease();

?>