class foo
{
    publc static function hello()
    {
        return "HI";
    }
}

foo::hello()

class Database extends PDO
{
    private static $_instance = null;

    private function __construct() {
        parent::__construct(APP_DB_DSN, APP_DB_USER, APP_DB_PASSWORD);
    }

    public static function getInstance() {
        if(!(self::$_instance instanceof Database)) {
            self::$_instance = new Database();
        }

        return self::$_instance;
    }
}

function hello() {
    return "HI";
}