<?php
require 'Classes/Calculator.php';
require 'Classes/OperatorInterface.php';
require 'Classes/Adder.php';
require 'Classes/Subtractor.php';

$c = new Calculator;

$c->setOperation(new Adder);
$c->calculate(10, 50);

echo $c->getResult();
?>

<?php
class Calculator{

    protected $result;

    protected $operation;

    public function setOperation(OperatorInterface $operation){
        $this->operation = $operation;
    }

    public function calculate(){
        foreach(func_get_args() as $number){
            $this->result = $this->operation->run($number, $this->$result);
        }
    }

    public function getResult(){
        return $this->result;
    }
}
?>

<?php
class Adder implements OperatorInterface{
    public function run($number, $result){
        return $result + $number;
    }
}
?>

<?php
interface OperatorInterface{
    public function run($number, $result);
}
?>