Change the boolean results

General Discussion
1

public function validate_boolean($value)
{
if ($value == “0” || $value == “1”) {
return true;
}

   if ($value == "") {
       return null;
   }

   return false;

}

this part of code is in the validator.php. I need that the boolean custom fileds show me NO or YES. At this moment, i obtain 0 and 1 as results. Where, in which(s) files i need modify?
Thanks in advance