How to inherit attribute for hidden/show form field

Create validation rules only for fields that are visible.
E.g. field10 and field11 are only visible if field4 has a value:

PHP Code:

if ($this->input->post('field4')) {
  $this->form_validation->set_rules('field10', 'Field 10', 'required' );
  $this->form_validation->set_rules('field11', 'Field 11', 'required' );
}