Class DB
{
    private static $instance;

    $dbUser = 'aoeu';
    $dbPass = 'aoeu';

    function __construct() {
        return connect_db($dbUser, $dbPass);
    }

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

Class DbChild extends DB
{
    public function articlesAll() {
        $query = "SELECT * FROM Articles";
        $stmt = self::getInstance()->query($query);
        $result = $stmt->fetch(PDO::FETCH_ASSOC);
        return $result;
    }
}