Form Validation

Something like this:

$this->form_validation->set_rules('inp', 'Foo', 'required');
$this->form_validation->set_rules('inp[]', 'Bar', 'required');

Though, I don't understand everybody's obsession with making their inputs arrays ...

If you want to receive an undefined count of inputs - you don't need to require at least one.
If one is mandatory and more are optional; then the first is not the same as the optional ones - use two separate inputs.
If you're trying to do 'user[name]' when 'user_name' works perfectly fine, then you're just doing it wrong.