discount.php

<?php

interface Discount
{
  /**
   * @return int
   */
  function getDiscountValue(Order $order);
}

class CalendarDiscount implements Discount
{
  function getDiscountValue(Order $order)
  {
    if ($order…)
      return 5;
    return 0;
  }
}

class Order {
  protected $discount = [];
  function applyDiscount(Discount $discount) {
    $value = $discount->getDiscountValue($this);
    if ($value) {
      $this->discount[] = $value;
    }
  }
  function getPrice() {
    return $this->price * (1 - array_sum($this->discount));
  }
}