Is it possible to access multiple object's property created from same class?

Is it possible?, I need to find difference between two object's property value created from same class. For example:

class Category {
   public $value1;
   public $value2;
   public $result;

   public function calc($input1, $input2){
      $this->value1 = $input1;
      $this->value1 = $input2;
      $this->result = $this->value1 * $this->value2;
   }
}

class SpecialCategory extends Category {
    public function calcDif() {
       $this->result = $current->result - $prior->result;
    }

}

$current = new Category();
$current->calc("10", "5");
$prior = new Category();
$prior->calc("5", "4");
$difference = new SpecialCategory();
$difference->calcDif();

echo $difference->result;

I want to find the difference between same property in difference object but created from same class, the error message is undefined variable in $this->result = $current->result - $prior->result;