Updates and fixes.

- Added the last open <?php tag
- Suggested a new error message, more fitting one in my opinion
- Updated error message for polish translation and a nickname typo :P

Also, I remember there was an issue if username is made of digits only, like the user: 314104778
By default, the query will treat it like user_id, not like a username, so there also has to be the type specified for query: `&type=string`, because while getting data from username made of digits, API gives no data.

And about the "Score needed from a level to another." option, I think it should work by calling the `ScoreLevelCalculator()` function with passing two parameters for `current level` and `target` like:
`Calculating score from level 60 to 65:`
```
$current_level_score = ScoreLevelCalculator(60); // example, replace with $_POST or $_GET
$target_level_score = ScoreLevelCalculator(65); // same

$thefinalresult = $target_level_score - $current_level_score;
```
~Which gives the difference of 5 levels and score needed for them. This has to be tested.