How to create IF->ELSE with input->post() with checkbox array?

PHP Code:

if($item){ //Run only when submit the form
  if ($item == NULL) { 


This will never work. If you haven't selected any checkbox, $item will be empty, so the line with "if ($item == NULL) {" will not be executed.
I assume that your form has a submit button, probably with the name "submit".

PHP Code:

$submit = $this->input->post('submit');
if ($submit) {
  $items = $this->input->post('service');
  if (! empty($items)) {
    foreach($items as $item) {
        //process the checkboxes that where checked
    }
  }
  else {
      // redirect
  }

}