Calculator(using include/require)

//Son.php file
<?php

  require 'Father.php';
  //include 'Father.php';

  class ChildClass extends ParentClass
  {
      private $sum,$sub,$mult,$div;

    function __construct($num1,$num2){
        $this->sum = $this->add($num1,$num2);
        $this->sub = $this->subtract($num1,$num2);
        $this->mult = $this->multiply($num1,$num2);
        $this->div = $this->divide($num1,$num2);
    }

    public function Display(){
       echo 'Summation is: '. $this->sum.'<br>';
       echo 'Subtract is: '. $this->sub.'<br>';
       echo 'Multiply is: '. $this->mult.'<br>';
       echo 'Division is: '. $this->div.'<br>';
    }

   }

   $myObject = new ChildClass(20,10);

   echo $myObject->Display();
?>

//Father.php file
<?php
  class ParentClass
   {
      /*private $num1;
      private $num2;*/

      protected function add($num1,$num2){
         /*$this->num1 = $num1;
         $this->num2 = $num2;*/
         return $num1 + $num2;
      }

      protected function subtract($num1,$num2){
        /* $this->num1 = $num1;
         $this->num2 = $num2;*/
         return $num1 - $num2;
      }

      protected function multiply($num1,$num2){
        /* $this->num1 = $num1;
         $this->num2 = $num2;*/
         return $num1 * $num2;
      }

      protected function divide($num1,$num2){
         /*$this->num1 = $num1;
         $this->num2 = $num2;*/
         return $num1 / $num2;
      }

   }
?>