An example using a Test Double Spy

TestLoggerSpy.php

<?php declare(strict_types=1);

use PHPUnit\Framework\TestCase;

interface Logger {
    public function log(string $message): void;
}

final class LoggerSpy implements Logger {
    public array $messages = [];

    public function log(string $message): void
    {
        $this->messages[] = $message;
    }
}

final class UserNotifier {
    private Logger $logger;

    public function __construct(Logger $logger)
    {
        $this->logger = $logger;
    }

    public function registerUser(User $user): void
    {
        $this->logger->log("Notifying the user {$user->name()}");
        // Doing other stuff...
    }
}

final class UserNotifierTest extends TestCase {
    public function testLogMessages(): void
    {
        $logger = new LoggerSpy();
        $notifier = new UserNotifier($logger);
        $notifier->registerUser(new User());

        $this->assertStringContainsString('Notifying the user', end($logger->messages));
    }
}