Pull Request 

please assign a default value for `$page_num`
so that when you call:
```php
pagination($thread_count, '');
```
you can instead call it like this:
```php 
pagination($thread_count);
```

Also, I think the default value for `$set_url` should be `$array` instead of `NULL` since you are also typecasting it to array. :smile:

Lastly po, please change the name from `$set_url` to more appropriate po. Based from the usage in this function, it seems like a parameter for the router `url()` :bow: