<?php

class Category {}

interface CategoryRepository {

    /**
     * @return Category[]
     */
    public function all(): array;

    public function save(Category $category);

}

class CategoryService {

    private $categoryRepository;

    public function __construct(CategoryRepository $categoryRepository) {
        $this->categoryRepository = $categoryRepository;
    }

    public function hardDomainLogic(Category $category) {
        $availableCategories = $this->categoryRepository->all();
        // ...
    }

}

class TestCategoryService {

    public function testHardDomainLogic() {
        $categoryA = new Category();
        $categoryB = new Category();
        $categoryC = new Category();

        $categoryRepository = $this->createCategoryRepository();
        $categoryRepository->save($categoryA);
        $categoryRepository->save($categoryB);

        $service = new CategoryService($categoryRepository);
        $service->hardDomainLogic($categoryC);
    }

    private function createCategoryRepository(): CategoryRepository {
        return new class implements CategoryRepository {

            private $categories = [];

            public function save(Category $category) {
                $this->categories[] = $category;
            }

            public function all(): array {
                return $this->categories;
            }
        };
    }

}

$test = new TestCategoryService();
$test->testHardDomainLogic();