Mocking Best Practices (PHPUnit & Mockery)

Also Known As

TL;DR

Explanation

Key mocking guidelines: mock interfaces rather than concrete classes (easier to substitute, forces DIP compliance). Distinguish stubs (return canned data — no assertions on calls) from mocks (assert that specific calls were made — for commands with side effects). Avoid over-mocking: if a test mocks 5 collaborators, the unit under test has too many dependencies — refactor. Avoid mocking types you don't own (3rd-party SDKs); wrap them in an adapter and mock the adapter. Use Mockery's shouldReceive for expressive expectations or PHPUnit's createMock. Test the interface contract, not the implementation — if an internal refactor breaks tests without changing behaviour, the tests were too tightly coupled to implementation.

Diagram

flowchart TD
    subgraph Test_Double_Types
        DUMMY[Dummy - passed but not used]
        STUB[Stub - returns canned values]
        SPY[Spy - records calls for assertions]
        MOCK2[Mock - pre-programmed expectations]
        FAKE[Fake - working implementation<br/>InMemoryRepository]
    end
    subgraph When_to_Mock
        MOCK_YES[External APIs<br/>Email and payment services<br/>Database in unit tests<br/>Time and random]
        MOCK_NO[Internal pure functions<br/>Value objects<br/>Domain logic]
    end
style FAKE fill:#238636,color:#fff
style MOCK2 fill:#1f6feb,color:#fff
style MOCK_YES fill:#238636,color:#fff
style MOCK_NO fill:#d29922,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Mocking types you do not own (third-party classes) — create a wrapper interface and mock that instead.
• Over-mocking — if your test needs ten mocks to run, the design has too many dependencies.
• Not verifying mock interactions when the side effect is the point — e.g. asserting the mailer was called.
• Using mocks in integration tests — integration tests should use real (or in-memory) implementations.

Avoid When

• Mocking value objects or pure functions — these have no side effects and should be used directly in tests.
• Mocking the system under test itself — you end up testing your mock, not your code.
• Over-mocking to the point where the test passes even when the real implementation is broken.
• Mocking concrete classes with no interface — it couples the test to implementation details and breaks on refactoring.

When To Use

• Isolating the unit under test from slow or non-deterministic dependencies like databases, clocks, and HTTP clients.
• Verifying that the unit calls a dependency with the correct arguments — interaction testing.
• Simulating error conditions (network timeout, DB failure) that are hard to reproduce with real dependencies.
• Replacing third-party services in unit tests to avoid network calls, costs, and rate limits.

Code Examples

// Mock verifying calls instead of outcomes:
$mailer = $this->createMock(Mailer::class);
$mailer->expects($this->once())->method('send'); // Verifies send was called
// But: was the right email sent? To the right address? With the right content?
// Test passes even if the email body is empty or wrong
// Better: capture arguments and assert on the actual email content

// Using Mockery (more expressive than PHPUnit mocks)
use Mockery;

public function test_order_service(): void {
    $mailer = Mockery::mock(MailerInterface::class);
    $mailer->shouldReceive('send')
           ->once()
           ->with(Mockery::type(OrderConfirmation::class));

    (new OrderService($mailer))->place($cart);
    Mockery::close(); // or use MockeryPHPUnitIntegration trait
}

// Don't mock what you don't own — wrap 3rd-party libs
// Don't mock value objects — use real instances
// Prefer fakes for complex dependencies (in-memory repository)

References

• ↗ https://docs.mockery.io/
• ↗ https://phpunit.de/manual/current/en/test-doubles.html

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