LoggerStub

LoggerStub.php

<?php

declare(strict_types=1);

namespace App;

use Psr\Log\AbstractLogger;

final class LoggerStub extends AbstractLogger
{
    /**
     * @var list<array{mixed, \Stringable|string, array}>
     */
    public array $logs = [];

    public function log($level, \Stringable|string $message, array $context = []): void
    {
        $this->logs[] = [$level, $message, $context];
    }
}

SomeService.php

<?php

declare(strict_types=1);

namespace App;

use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;

final readonly class SomeService
{
    public function __construct(
        private LoggerInterface $logger = new NullLogger(),
    ) {}

    public function doStuff(): void
    {
        // ...
        $this->logger->info('I did it!', ['key' => 'value']);
        // ...
    }
}

SomeServiceTest.php

<?php

declare(strict_types=1);

namespace App;

use PHPUnit\Framework\Attributes\CoversClass;
use PHPUnit\Framework\TestCase;
use Psr\Log\LogLevel;

#[CoversClass(SomeService::class)]
final class SomeServiceTest extends TestCase
{
    public function testItLogs(): void
    {
        $logger = new LoggerStub();
        $service = new SomeService($logger);

        $service->doStuff();

        self::assertSame(
            [[LogLevel::INFO, 'I did it!', ['key' => 'value']]],
            $logger->logs,
        );
    }
}