Create Interface
Strictly speaking, you do not have to create an interface for each Dto. However, if you are working on a large and complex system, then it might be a good idea. It will help you to keep a clean design throughout your system's architecture.
Below you will see a simplified interface for a Person
Dto. It might not conform to your needs, but it exemplifies how Dto interfaces might look like.
namespace Acme;
interface Person
{
public function setName(?string $name);
public function getName() : ?string;
public function setAge(?int $age);
public function getAge() : ?int;
}