if (isset($_POST['pseudo']) && isset($_POST['password'])) {
    $hash = $dbUser->getPasswordByName($_POST['pseudo']);
    $res = array();                                    // line 15

    if (password_verify($_POST['password'], $hash)) {
                                                       // line 20
        $res = $dbSession->addSession(session_id());

        $_SESSION["auth"] = "true";

        var_dump(is_scalar($_SESSION));                // false
        var_dump(is_scalar($res));                     // true <--

        $_SESSION["pseudo"] = $_POST['pseudo'];
        $res["auth"] = "true";                         // line 29 <--

    } else {
        $_SESSION["auth"] = "false";
        $res["auth"] = "false";
    }

    echo json_encode($res);
    return 0;
}