It's common to encounter infrastructural dependencies when modeling a Domain Service  — for example, in the case where an authentication mechanism that handles password hashing is required. In this instance, you could use a Separated Interface, which allows for multiple hashing mechanisms to be defined. Using this pattern still provides you with a clear Separation of Concerns between the Domain and the Infrastructure:

namespace Ddd\Auth\Domain\Model;

interface SignUp
{
    public function execute($aUsername, $aPassword);
}

Using the preceding interface found in the Domain, we could create an implementation in the Infrastructure layer, like the following:

namespace Ddd\Auth\Infrastructure\Authentication;

class DefaultHashingSignUp implements Ddd\Auth\Domain\Model\SignUp
{
    private $userRepository;

    public function __construct(UserRepository $userRepository)
    {
        $this->userRepository = $userRepository;
    }

    public function...