$this->form_validation->set_rules('my_field[]', 'my_field', array(array('my_function['.$extra_parameter.']', function($value, $extra_parameter) { // My code }) ));
$this->form_validation->set_rules('my_field[]', 'my_field', array(array('my_function', function($value) use ($extra_parameter) { // My code }) ));