<?php
class Order
{
    private $amount;
    private $tax;

    public function __construct($amount, $tax)
    {
        $this->amount = $amount;
        $this->tax = $tax;
    }

    /**
     * @return mixed
     */
    public function getAmount()
    {
        return $this->amount;
    }

    /**
     * @param mixed $amount
     */
    public function setAmount($amount)
    {
        $this->amount = $amount;
    }

    /**
     * @return mixed
     */
    public function getTax()
    {
        return $this->tax;
    }

    /**
     * @param mixed $tax
     */
    public function setTax($tax)
    {
        $this->tax = $tax;
    }
}

class CalculatorService
{
    public function computeTotal(Order $order)
    {
        return $order->getTax() * $order->getAmount();
    }
}







$order = new Order(100, 0.2);
$calculator = new CalculatorService();

echo $calculator->computeTotal($order);