Test Doubles

Also Known As

TL;DR

Explanation

Stub: returns predefined values, no assertions. Mock: verifies interactions (was a method called, how many times, with what args). Spy: records calls for later assertion. Fake: a simplified but working implementation (e.g. in-memory database). Dummy: passed but never used, just to satisfy a parameter. PHPUnit provides mock objects via createMock() and createStub(). Mockery offers a more expressive DSL. Overusing mocks leads to tests that pass even when the real integration is broken.

Common Misconception

Why It Matters

Common Mistakes

• Mocking third-party libraries directly — wrap them in an adapter and mock the adapter instead.
• Using mocks where a fake (in-memory implementation) would give more realistic test coverage.
• Asserting on mock interactions instead of observable outputs — testing how rather than what.

Avoid When

• Do not mock value objects or simple data structures — instantiate them directly.
• Avoid mocking third-party libraries — wrap them in your own interface and mock that instead.

When To Use

• Use stubs when you need a dependency to return a specific value without caring how it was called.
• Use mocks when the interaction itself (method called, args, times) is the behaviour under test.

Code Examples

// Mocking a third-party class directly — fragile, couples to implementation
$guzzle = $this->createMock(\GuzzleHttp\Client::class);
$guzzle->method('request')->willReturn(new Response(200, [], 'ok'));
// Better: mock your own HttpClientInterface that wraps Guzzle

// Stub: provide a return value
$mailer = $this->createStub(MailerInterface::class);
$mailer->method('send')->willReturn(true);

// Mock: verify interaction
$mailer = $this->createMock(MailerInterface::class);
$mailer->expects($this->once())
       ->method('send')
       ->with($this->equalTo('user@example.com'));

$service = new UserService($mailer);
$service->registerUser('user@example.com');

References

• ↗ https://phpunit.de/documentation.html

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