I could not agree more with Brent when he says concerning the "new in initializers"^[1] feature:

PHP 8.1 adds a feature that might seem like a small detail, but one that I think will have a significant day-by-day impact on many people.

When I see this new feature, lots of places that use Dependency Injection^[3] come to my focus as candidates to be impacted, such as application or infrastructure service classes. As a result, we will write a much cleaner and leaner code without giving up on good practices to write modular, maintainable and testable software.

The Dependency Inversion Principle^[4] gives us decoupling powers. But we know that many classes will receive the same concrete implementation most of (if not all) the time.

So this is very common to see some variation of this code:

$someDependencyToBeInjected = FactoryClass::create();
$someService = new SomeServiceClass($someDependencyToBeInjected);

Important note: I will ignore for now Service Containers and frameworks features that deal with service instantiation, auto wiring, etc.

Think of a database query service class: you depend on a connection object. Every time you need to instantiate your database service class, you need to prepare the connection dependency and inject it at the service class. Database connections are a great example when you use the same concrete implementation more than 90% of the time.

The same applies to a Service class that you use to handle business logic and depends on QueryService and CommandHandler interfaces to do its job.

Before PHP 8.1 we have code like this:

// service class to apply business logic
// most standard
class DefaultLeadRecordService implements LeadRecordService
{
    public function __construct(
        private LeadQueryService $queryService,
        private LeadCommandHandler $commandHandler
    ) {
    }
}

// infrastructure class to match the interface -- a DBAL concrete class
// sneakily allowing a "default" value, but also open to Dependency Injection
// but not that great
class DbalLeadQueryService implements LeadQueryService
{
    public function __construct(private ?Connection $connection = null)
    {
        if (!$this->connection) {
            $this->connection = Core::getConnection();
        }
    }
}

// instantiation would be something like -- given you have $connection already instantiated
$connection =  \Doctrine\DBAL\DriverManager::getConnection($connectionParams, $config);

$service = new \Blog\Application\DefaultLeadRecordService(
    new \Blog\Infrastructure\DbalLeadQueryService($connection),
    new \Blog\Infrastructure\DbalLeadCommandHandler($connection),
);

// if we allow construct get default value
$service = new \Blog\Application\DefaultLeadRecordService(
    new \Blog\Infrastructure\DbalLeadQueryService(),
    new \Blog\Infrastructure\DbalLeadCommandHandler(),
);

While on PHP 8.1, you will be able to write it like so:

class DefaultLeadRecordService implements LeadRecordService
{
    public function __construct(
        private LeadQueryService $queryService = new DbalLeadQueryService(),
        private LeadCommandHandler $commandHandler = new DbalLeadCommandHandler()
    ) {
    }
}

// we see there is still room for new features here
// still not that great
class DbalLeadQueryService implements LeadQueryService
{
    public function __construct(private ?Connection $connection = null)
    {
        // waiting when new initializers feature allows static function as default parameters
        if (!$this->connection) {
            $this->connection = Core::getConnection();
        }
    }
}

$service = new \Blog\Application\DefaultLeadRecordService();

One liner! That saves a lot of typing, and the code remains very well structured. This is the type of significant impact we will have on our day-to-day work. We will write a more straightforward, robust and meaningful code, and we will ship features faster with high-quality code.

If you want to see a full implementation of this, check the code at https://github.com/rafaelbernard/blog-php-81-new-initializers

Test, our faithful friend

Writing tests is a must-have for any repository where quality is a requirement. However, the "New in initializers" feature does not force us to give up on a complete suite of tests. We still have all powers of unit or integration tests.

For application code, we would write unit tests and all the expectations for concrete dependencies:

<?php

namespace Test\Unit\Blog\Application;

use Blog\Application\DefaultLeadRecordService;
use Blog\Domain\LeadCommandHandler;
use Blog\Domain\LeadQueryService;
use PHPUnit\Framework\MockObject\MockObject;
use Test\TestCase;

class DefaultLeadRecordServiceTest extends TestCase
{
    private const EMAIL = '[email protected]';

    private LeadQueryService|MockObject $leadQueryServiceMock;
    private LeadCommandHandler|MockObject $leadCommandHandlerMock;

    private DefaultLeadRecordService $service;

    protected function setUp(): void
    {
        parent::setUp();

        $this->leadQueryServiceMock = $this->getMockBuilder(LeadQueryService::class)->getMock();
        $this->leadCommandHandlerMock = $this->getMockBuilder(LeadCommandHandler::class)->getMock();

        $this->service = new DefaultLeadRecordService($this->leadQueryServiceMock, $this->leadCommandHandlerMock);
    }

    public function testCanAdd()
    {
        $this->leadQueryServiceMock
            ->expects(self::once())
            ->method('getByEmail')
            ->with(self::EMAIL)
            ->willReturn(false);

        $this->leadCommandHandlerMock
            ->expects(self::once())
            ->method('add')
            ->with(self::EMAIL)
            ->willReturn(1);

        $result = $this->service->add(self::EMAIL);

        self::assertEquals(1, $result);
    }

    public function testAddExistentReturnsFalse()
    {
        $this->leadQueryServiceMock
            ->expects(self::once())
            ->method('getByEmail')
            ->with(self::EMAIL)
            ->willReturn(['email' => self::EMAIL]);

        $this->leadCommandHandlerMock
            ->expects(self::never())
            ->method('add');

        $result = $this->service->add(self::EMAIL);

        self::assertFalse($result);
    }

    public function testCanGetAll()
    {
        $unsorted = [
            ['email' => '[email protected]'],
            ['email' => '[email protected]'],
            ['email' => '[email protected]'],
        ];

        $this->leadQueryServiceMock
            ->expects(self::once())
            ->method('getAll')
            ->willReturn($unsorted);

        $fetched = $this->service->getAll();

        $expected = $unsorted;
        asort($expected);

        self::assertEquals($expected, $fetched);
    }
}

Integration tests can be written for infrastructure code. For instance, we can use an SQLite database file to assert the logic for database operations.

Be aware that I am creating an SQLite temp database file on-demand for each test execution with $this->databaseFilePath = '/tmp/test-' . time(); and, thanks to the Dbal library, we can be confident that operations could work for any database.

-> It is highly recommended that, as an alternative, create a container with a seeded database that is compatible with your production database system.

<?php

namespace Test\Integration\Blog\Infrastructure;

use Blog\Infrastructure\DbalLeadQueryService;
use Doctrine\DBAL\Connection;
use Faker\Factory;
use Faker\Generator;
use Test\TestCase;

class DbalLeadQueryServiceTest extends TestCase
{
    private string $databaseFilePath;

    private Generator $faker;
    private Connection $connection;

    private DbalLeadQueryService $service;

    public function testCanGetAll()
    {
        $this->addEmail($email1 = $this->faker->email());
        $this->addEmail($email2 = $this->faker->email());
        $this->addEmail($email3 = $this->faker->email());

        $expected = [
            ['email' => $email1],
            ['email' => $email2],
            ['email' => $email3],
        ];

        $fetched = $this->service->getAll();

        self::assertEquals($expected, $fetched);
    }

    protected function setUp(): void
    {
        parent::setUp();

        $this->faker = Factory::create();

        $this->createLeadTable();

        $this->service = new DbalLeadQueryService($this->connection());
    }

    protected function tearDown(): void
    {
        parent::tearDown();

        $this->dropDatabase();
    }

    private function connection(): Connection
    {
        if (!isset($this->connection)) {
            $this->databaseFilePath = '/tmp/test-' . time();

            $config = new \Doctrine\DBAL\Configuration();
            $connectionParams = [
                'url' => "sqlite:///{$this->databaseFilePath}",
            ];

            $this->connection = DriverManager::getConnection($connectionParams, $config);
        }

        return $this->connection;
    }

    private function dropDatabase()
    {
        @unlink($this->databaseFilePath);
    }

    private function createLeadTable(): void
    {
        $this->connection()->executeQuery('CREATE TABLE IF NOT EXISTS leads ( email VARCHAR )');
    }

    private function addEmail(string $email): int
    {
        return $this->connection()->insert('leads', ['email' => $email]);
    }
}

Conclusion

PHP is evolving very quickly, with new features that enable more quality software, help developers and is even more committed to the fact that most of the web run flavours of PHP code. New features improve readability, software architecture, test coverage and performance. Those are all proof of a mature and live language.

Upgrade to PHP 8.1 and use "new in initializers" as soon as possible. You will not regret it.

If there is something you want to discuss more, let me know in the comments.

Links:

1. New in initializers RFC
2. Road to PHP 8.1
3. Dependency Injection
4. Dependency inversion principle
5. Interface segregation principle
6. Solid relevance
7. SOLID principles