PHPUnit – unit tests, mocks, data providers, testing in Magento 2

Unit testing in PHP projects is a topic most developers know in theory but keep putting off until “when there is time”. The time never comes. I show how to get started with PHPUnit, how to write tests that actually check something meaningful, and how to mock dependencies – with examples that look like real production code.

Installation and first test

composer require --dev phpunit/phpunit

# Run tests
vendor/bin/phpunit tests/

# With code coverage (requires Xdebug or PCOV)
vendor/bin/phpunit --coverage-html coverage/ tests/

<?php

declare(strict_types=1);

use PHPUnit\Framework\TestCase;

class PriceCalculatorTest extends TestCase
{
    private PriceCalculator $calculator;

    protected function setUp(): void
    {
        $this->calculator = new PriceCalculator();
    }

    public function testCalculatesTotalWithTax(): void
    {
        $result = $this->calculator->calculateTotal(100.0, 0.23);
        $this->assertEquals(123.0, $result);
    }

    public function testReturnsPriceWhenTaxIsZero(): void
    {
        $result = $this->calculator->calculateTotal(100.0, 0.0);
        $this->assertEquals(100.0, $result);
    }

    public function testThrowsOnNegativePrice(): void
    {
        $this->expectException(\InvalidArgumentException::class);
        $this->calculator->calculateTotal(-10.0, 0.23);
    }
}

Mocking dependencies

<?php

use PHPUnit\Framework\TestCase;
use PHPUnit\Framework\MockObject\MockObject;

class OrderServiceTest extends TestCase
{
    private MockObject $orderRepository;
    private MockObject $emailNotifier;
    private OrderService $service;

    protected function setUp(): void
    {
        // createMock() generates a class that implements the interface
        // but does nothing by default
        $this->orderRepository = $this->createMock(OrderRepositoryInterface::class);
        $this->emailNotifier   = $this->createMock(EmailNotifierInterface::class);

        $this->service = new OrderService(
            $this->orderRepository,
            $this->emailNotifier
        );
    }

    public function testCancelsOrderAndSendsEmail(): void
    {
        $order = $this->createMock(OrderInterface::class);
        $order->method('getId')->willReturn(42);
        $order->method('getCustomerEmail')->willReturn('customer@example.com');

        // Define what the mock should return
        $this->orderRepository
            ->method('getById')
            ->with(42)
            ->willReturn($order);

        // Assert the mock is called with specific arguments
        $order->expects($this->once())
            ->method('setStatus')
            ->with('cancelled');

        $this->orderRepository->expects($this->once())
            ->method('save')
            ->with($order);

        $this->emailNotifier->expects($this->once())
            ->method('sendCancellationEmail')
            ->with('customer@example.com');

        $this->service->cancelOrder(42);
    }
}

Data providers – test multiple inputs

<?php

use PHPUnit\Framework\TestCase;
use PHPUnit\Framework\Attributes\DataProvider;

class PriceCalculatorTest extends TestCase
{
    // Data provider - returns sets of [input, expected]
    public static function priceProvider(): array
    {
        return [
            'zero tax'        => [100.0, 0.0,  100.0],
            '23% VAT'         => [100.0, 0.23, 123.0],
            '8% reduced rate' => [100.0, 0.08, 108.0],
            'small amount'    => [0.01,  0.23,  0.0123],
        ];
    }

    #[DataProvider('priceProvider')]
    public function testCalculatesCorrectly(float $price, float $tax, float $expected): void
    {
        $result = (new PriceCalculator())->calculateTotal($price, $tax);
        $this->assertEqualsWithDelta($expected, $result, 0.001);
    }
}

Testing in Magento 2 – integration tests

<?php

use Magento\TestFramework\Helper\Bootstrap;
use PHPUnit\Framework\TestCase;

class ProductRepositoryTest extends TestCase
{
    private \Magento\Catalog\Api\ProductRepositoryInterface $repository;

    protected function setUp(): void
    {
        // Bootstrap Magento Object Manager in tests
        $objectManager    = Bootstrap::getObjectManager();
        $this->repository = $objectManager->get(
            \Magento\Catalog\Api\ProductRepositoryInterface::class
        );
    }

    /**
     * @magentoDataFixture Magento/Catalog/_files/product_simple.php
     */
    public function testGetBySkuReturnsProduct(): void
    {
        $product = $this->repository->get('simple');
        $this->assertEquals('simple', $product->getSku());
        $this->assertEquals(1, $product->getStatus());
    }
}

# Run Magento integration tests
cd dev/tests/integration
../../../vendor/bin/phpunit --config phpunit.xml \
    testsuite/Magento/Catalog/Model/ProductRepositoryTest.php

phpunit.xml – configuration

<?xml version="1.0" encoding="UTF-8"?>
<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
         bootstrap="vendor/autoload.php"
         colors="true"
         stopOnFailure="false">

    <testsuites>
        <testsuite name="Unit">
            <directory>tests/Unit</directory>
        </testsuite>
        <testsuite name="Integration">
            <directory>tests/Integration</directory>
        </testsuite>
    </testsuites>

    <source>
        <include>
            <directory suffix=".php">src</directory>
        </include>
    </source>
</phpunit>

Summary

Unit tests pay off even in small projects – you catch regressions in minutes instead of after a client call. The key is to start simple: test pure functions and service classes that have a clear, mockable dependency interface. In Magento 2 the Service Contracts architecture with Repository interfaces makes this straightforward – mock the repository, test the service logic.