<?php

// your code goes here

class Application {
    static $db_connected = false;
}

class Database extends Application {
    function connect() {
        if(!static::$db_connected) {
            static::$db_connected = true;
        }
    }
}

$app = new Application;
$db = new Database;

var_dump(Application::$db_connected);
var_dump(Database::$db_connected);

$db->connect();

var_dump(Application::$db_connected);
var_dump(Database::$db_connected);