class SessionManager{
   /* MUSI BYT INICIALIZOVANO V DETECH*/
   private $db_user_id; # select pro overeni, jestli username a password existuje
   /* KONEC POVINNE DEFINICE V DETECH*/
   private $session_lifespan = 3600; //doba platnosti relace v sekundach
   private $is_logged = false;
   private $db;

   public function __construct($db){
      $this->db = $db;
      //nastaveni doby platnosti cookie
      session_set_cookie_params($this->session_lifespan);
      session_start();
        ...
   }

   public function login($username, $password){
      $escaped_username = pg_escape_literal($username);
      $escaped_password = pg_escape_literal($password);

      $result = pg_query($db_user_id);

      //pokud existuje user a shoduje se heslo = databaze vraci alespoň jeden row a pak povazujeme za opravneny pristup
      if(pg_num_rows($result) > 0){
            $row = pg_fetch_array($result);
            $_SESSION["user_id"] = $row["user_id"];
            $this->logged_in = true;
            return(true);
         }
         else{
            session_destroy();
            return(false);
         }
   }

class SessionManagerWebsite extends SessionManager{
    // zde chci ulozit select, ktery se pouze ve fci rodice - login();
    $db_user_id = "SELECT FROM ....";
}