discussion.php

<?php

class Discussion
{
    private $id;

    private function __construct(DiscussionId $discussionId)
    {
        $this->id = $discussionId;
    }
    
    public function equals(Discussion $discussion)
    {
        return $this->id->equals($discussion->id);
    }
}

discussionId.php

<?php

class DiscussionId
{
    private $id;

    public function __construct($d)
    {
        $this->id = $id;
    }
    
    public function equals(DiscussionId $discussionId)
    {
        return $this->id === $discussionId->id;
    }
}

testing.md

The problem is : What is the proper way to test discussion's method equals with PhpSpec.

You ended up with 2 very good examples in my book 👍

The thing which was bothering us was that we weren't testing that the logic was delegated to DiscussionId's equals method.

I generally am not interested in communication between two values, I am interested in communication between two roles. In your case, you don't have a role - you have a very specific value representation - DiscussionId. DiscussionId is not a collaborator, it is a value (an object incorporating a singular value with a set of behaviour surrounding it). I wouldn't stub or mock DiscussionId as I wouldn't stub or mock array or collection - there's no collaboration with ValueObject's, you just use them as you would use integers or strings in your tests.

I didn't think of a value object as a value instead of a collaborator but yes it makes sens. (Didn't see the big V before object, duh...)

I'm curious, what are the two examples ?
I guess clarifying that a VO is a value as native value would be is one of them but I can't figure what the second would be.