<?php
declare(strict_types=1);

use DateTimeImmutable;
use ReflectionClass;
use PHPUnit\Framework\TestCase;
use App\Entities\Entity;

class SomeIntegrationTest extends TestCase
{
    public function test_something_related_to_entity_creation_date()
    {
        $entity = new Entity();

        $reflection = new ReflectionClass($entity);
        $property = $reflection->getProperty('creationDate');
        $property->setAccessible(true);
        $property->setValue($entity, new DateTimeImmutable('1 year ago');
        
        // Now $entity->getCreationDate() returns '1 year ago'. Your fixture is ready!
    }
}