itmo-php-course/Koterov-php-training/methods_of_classes/static.php

<?php

class Counter
{
    private static $count = 0;

    public function __construct()
    {
        self::$count++;
    }

    public function __destruct()
    {
        self::$count--;
    }
    public static function getCount(): int   
    {
        return self::$count;
    }
}