Unit Testing with PHPUnit

PHPUnit is the standard PHP test framework (Pest builds a friendlier syntax on top). Tests extend TestCase, use assertions like assertSame(), and since PHPUnit 10/11 use attributes (#[Test], #[DataProvider], #[CoversClass]) instead of docblock annotations. Follow Arrange-Act-Assert, test behavior through public methods, and use data providers for tables of cases.

<?php
use PHPUnit\Framework\Attributes\DataProvider;
use PHPUnit\Framework\Attributes\Test;
use PHPUnit\Framework\TestCase;

final class MoneyTest extends TestCase
{
    #[Test]
    public function it_adds_amounts_in_the_same_currency(): void
    {
        $total = (new Money(1000, 'USD'))->add(new Money(250, 'USD'));
        $this->assertSame(1250, $total->amount);
    }

    #[Test]
    public function it_rejects_currency_mismatch(): void
    {
        $this->expectException(LogicException::class);
        (new Money(1, 'USD'))->add(new Money(1, 'EUR'));
    }

    public static function invalidAmounts(): iterable
    {
        yield 'negative' => [-1];
        yield 'very negative' => [-1000];
    }

    #[Test, DataProvider('invalidAmounts')]
    public function it_rejects_negative_amounts(int $amount): void
    {
        $this->expectException(InvalidArgumentException::class);
        new Money($amount, 'USD');
    }
}

Common pitfalls

• Using assertEquals where assertSame is meant (loose comparison).
• Data providers must be public static in PHPUnit 10+.
• Testing private methods directly instead of behavior.

Key points

• assertSame for strict checks
• Attributes replace annotations
• Data providers for case tables

Further reading: PHPUnit documentation