$db_host = 'localhost';
$db_user = 'root';
$db_pass = '';
$db_name = 'szkola';

$conn = mysqli_connect($db_host, $db_user, $db_pass, $db_name)
        or die("Failed to connect");

$login = mysqli_real_escape_string($conn, $_POST['login']);
$passWord = mysqli_real_escape_string($conn, $_POST['password']);
$firstName = mysqli_real_escape_string($conn, $_POST['firstName']);
$lastName = mysqli_real_escape_string($conn, $_POST['lastName']);

$firstQuerry = "INSERT INTO `uczen` (`uczen_id`, `imie`, `nazwisko`) VALUES (NULL, '$firstName', '$lastName');";
$getId = mysqli_query($conn, "SELECT * FROM uczen WHERE imie='$firstName' AND nazwisko='$lastName'")
            or die("Failed to querry database " . mysqli_error($conn));
$rows = mysqli_fetch_array($getId);
$id = $rows['uczen_id'];
$id = intval($id);
print $id;
$secondQuerry = "INSERT INTO `rejestracja` (`uczen_id`, `login`, `haslo`) VALUES ('$id', '$login', '$passWord');";
if(!mysqli_query($conn, $firstQuerry)){
    echo "Error " .mysqli_error($conn);
}else{
    if(!mysqli_query($conn, $secondQuerry)){
        echo "Error " .mysqli_error($conn);
    }else{
        header("Location: rejestracja.php?msg=1");
    }
}