Author

class Author implements ValueObject {

	/**
	 * @var string
	 */
	protected $memberId;
	
	// constructor
	
	/**
	 * @param string $topic
	 * @return Thread
	 */
	public function openThread($topic) {
		return Thread::open($this->memberId, $topic);
	}
	
	// getter

}

MemberAdapter

class MemberAdapter implements Singleton {

	/**
	 * Injected with DI
	 * @var Membership\MemberRepository
	 */
	protected $memberRepository;
	
	/**
	 * @param string $memberId
	 * @return NULL|Author
	 */
	public function toAuthor($memberId) {
		$member = $this->memberRepository->findById($memberId);
		
		return ($member !== NULL) ? new Author($memberId) : NULL;
	}

}

Thread

class Thread implements AggregateRoot {
	
	/**
	 * @var string
	 */
	protected $threadId;
	
	/**
	 * @var Author
	 */
	protected $author;
	
	/**
	 * @var string
	 */
	protected $topic;
	
	static public function open(Author $author, $topic) {...
	
}

ThreadService

// Application Service
class ThreadService implements Singleton {

	// injected MemberAdapter
	
	// inject ThreadRepository

	/**
	 * @param string $memberId
	 * @param string $topic
	 * @return string Identifier of the opened thread
	 */
	public function openThread($memberId, $topic) {
		$author = $this->memberAdapter->toAuthor($memberId);
		
		if ($author === NULL) {
			throw new UnknownAuthorException(..
		}
		
		//$thread = Thread::open($author->getMemberId(), $topic);
		$thread = $author->openThread($topic);
		
		$this->threadRepository->save($thread);
		return $thread->getThreadId();
	}

}