Test double — any object standing in for a real dependency in a test

<?php
// THE FIVE TYPES illustrated:

// 1. DUMMY — passed but never used
class OrderServiceTest extends \PHPUnit\Framework\TestCase
{
    public function test_order_total(): void
    {
        $dummyLogger = $this->createMock(LoggerInterface::class); // never called, just fills the param
        $realRepo    = new InMemoryOrderRepository();

        $service = new OrderService($realRepo, $dummyLogger);
        // Test logic that never touches the logger
    }
}

// 2. STUB — returns hardcoded values, no expectation on calls
$stubGateway = $this->createMock(PaymentGateway::class);
$stubGateway->method('charge')
            ->willReturn(new PaymentResult(success: true, transactionId: 'tx_123'));
// stub returns the same value regardless of what args are passed
// We don't care if it's called once, twice, or not at all

// 3. FAKE — real but simplified implementation
class FakePaymentGateway implements PaymentGateway
{
    private array $charges = [];

    public function charge(int $amount, string $currency, string $token): PaymentResult
    {
        // Real logic but no actual Stripe call
        if ($token === 'fail') return new PaymentResult(success: false, transactionId: null);
        $id = 'fake_' . uniqid();
        $this->charges[] = compact('amount', 'currency', 'id');
        return new PaymentResult(success: true, transactionId: $id);
    }

    public function getCharges(): array { return $this->charges; }
}

// 4. SPY — records calls, assert after the fact
$spyLogger = $this->createMock(LoggerInterface::class);
// Run the code
$service->createOrder([...]);
// Then assert what was logged
$spyLogger->expects($this->atLeastOnce())->method('log'); // verify it was called

// 5. MOCK — expectations set BEFORE the call
$mockMailer = $this->createMock(MailerInterface::class);
$mockMailer->expects($this->once())       // EXACTLY once
           ->method('send')
           ->with($this->isInstanceOf(OrderConfirmation::class)); // specific argument expected

$service->createOrder([...]); // runs — if send() not called exactly once → test fails

// LARAVEL FAKES — built-in test doubles
\Mail::fake();
\Event::fake();
\Queue::fake();
\Storage::fake('s3');
\Http::fake(['https://api.stripe.com/*' => \Http::response(['id' => 'ch_123'], 200)]);

// After running code that should have triggered those:
\Mail::assertSent(OrderConfirmation::class);
\Event::assertDispatched(OrderPlaced::class);
\Queue::assertPushed(ProcessPaymentJob::class);
\Http::assertSent(fn($request) => str_contains($request->url(), 'stripe.com'));