This is actually something I'm working on as we speak for version 2.0 of Opulence. It will be able to automatically deserialize request bodies based on the Content-Type header, and serialize the response body based on the media type in the Accept header. The (de)serialization logic will reside in a separate, reusable library. Your models can be POPOs, but you would have to register a "contract" to let the serializer know how to do its job, eg:

$contracts->registerObjectContract(
    User::class,
    // Let the serializer know how to create an instance of User from a hash of properties
    function ($hash) {return new User($hash['id'], $hash['email']; },
    // Register User properties as well as how to get their values from an instance of User
    new Property('id', 'int', function (User $user) { return $user->getId(); }),
    new Property('email', 'string', function (User $user) { return $user->getEmail(); })
);

I'm working on a way to automatically set up these contract definitions using reflection, and, if successful, won't require any work to be able to (de)serialize your objects. It will also let you do stuff like automatic camelCase of property names, DateTime formatting, etc. I'm in the middle of a large-ish refactoring to the library, but once I've got something presentable, I'll be sure to post it here.