nullCheck.php

// Adapted from Code Smell: Null Check, Joe Eames https://medium.com/thinkster-io/code-smell-null-check-a0c4851fafbf

<?php

class DiscountCalculator {
    // ...

    public function getDiscount(string $customerId) {
        $customer = $this->customerRepository->findBy($customerId);
        if($customer == null) {
            return self::DEFAULT_DISCOUNT;
        } 
        return $this->$customer->getEarnedDiscount();
    }  
}

class Orders {
    // ...

    public function processOrder(string $customerId, Order $order) {
        $customer = $this->customerRepository->findBy($customerId);
        if($customer != null) {
            $customer->addToOrdersHistory($order);
        } 
        // more logic ...
    }
}

//--------------------------------------------------------------------
//--------------------------------------------------------------------

<?php

interface Customer {
  public function getEarnedDiscount(): float;

  public function addToOrdersHistory(Order $order): void;
}

// some Customer's implementations
// ...

// Customer's null implementation
class NotFoundCustomer implements Customer {
  private float DEFAULT_DISCOUNT = 1.0;

  //...
  public function getEarnedDiscount(): float {
    return $this->DEFAULT_DISCOUNT;
  }

  public function addToOrdersHistory(Order $order): void {

  }
}

class DiscountCalculator {
  private CustomerRepository $customerRepository;

  function __construct(CustomerRepository $customerRepository) {
    $this->customerRepository = $customerRepository;
  }

  // ...
  public function getDiscount(string $customerId): float {
    $customer = $this->customerRepository->findBy($customerId);
    return $customer->getEarnedDiscount();
  }
}

class Orders {
  private CustomerRepository $customerRepository;

  function __construct(CustomerRepository $customerRepository) {
    $this->customerRepository = $customerRepository;
  }

  // ...
  public function processOrder(string $customerId, Order $order): void {
    $customer = $this->customerRepository->findBy($customerId);
    $customer->addToOrdersHistory($order);

    // more logic ...
  }
}