Unit Testing

Also Known As

TL;DR

Explanation

Unit tests isolate a single class or function, replacing real dependencies (databases, HTTP clients, file systems) with test doubles (mocks, stubs, fakes). They run fast (no I/O), pinpoint failures precisely, and are the foundation of test-driven development. PHPUnit is the standard PHP unit testing framework. Well-designed code (injected dependencies, small classes) is easy to unit test — if a class is hard to test, that's a signal its design needs improvement. Aim for high coverage of business logic, edge cases, and error paths.

Common Misconception

Why It Matters

Common Mistakes

• Tests that test multiple units — failing tests give no indication of which unit is broken.
• Tests with no assertions — they pass always and provide false confidence.
• Tests tightly coupled to implementation details — refactoring breaks tests even when behaviour is correct.
• No tests for edge cases (empty input, null, zero, max values) — bugs live in boundaries.

Code Examples

// Test with no assertion — always passes:
public function testProcess(): void {
    $service = new OrderService();
    $service->process($order); // Called but nothing asserted
    // PHPUnit reports: 1 test, 0 assertions — misleadingly green
}

// With assertions:
public function testProcess(): void {
    $result = $service->process($order);
    $this->assertSame('processed', $result->status);
    $this->assertTrue($result->emailSent);
}

use PHPUnit\Framework\TestCase;

class MoneyTest extends TestCase {
    public function test_add_same_currency(): void {
        $a = new Money(100, 'GBP');
        $b = new Money(50, 'GBP');
        $this->assertEquals(new Money(150, 'GBP'), $a->add($b));
    }

    public function test_add_different_currency_throws(): void {
        $this->expectException(CurrencyMismatch::class);
        (new Money(100, 'GBP'))->add(new Money(100, 'USD'));
    }
}

References

• ↗ https://phpunit.de/
• ↗ https://martinfowler.com/bliki/UnitTest.html

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