How get value for unchecked checkbox in checkbox elements when form posted?

Question

I have a form like below :

Answer 1

                                                                  
Option 1
Option 2
Option 3
Option 4                                   

//php code in function called on form submit       
  public function addOptions(Request $request)
   {
    $option = array('1' => 0,'2'=>0,'3'=>0,'4'=>0 );
    $option[$request->correct] = 1;
    return $option;
   }