php 简单的工程模式 实现加法操作

class A{
     protected $Num1;
     protected $Num2;
    public  $result;
    public function getNum1(){
        return $this->Num1;
    }
    public function setNum1($Num1){
       return $this->Num1=$Num1;

    }
    public function getNum2(){
        return $this->Num2;
    }
    public function setNum2($Num2){
        return $this->Num2 = $Num2;

    }
    public function get_result(){
        return null;
    }

}
class Add extends A
{
    public function get_result()
    {
        return $this->Num1+$this->Num2;
    }
}
class Faction{

    public function getR($re){
        if($re=="+") {
            $com = new Add();
            return $com;
        }

    }
}
$model = new Faction();

$m = $model->getR("+");
$m->setNum1(1);
$m->setNum2(27777);

print_r($m->get_result());exit;