The 'feature filters' described here are an implementation of the Interface Segregation Principle, or to put it simply, keeping interfaces focused on one specific task:

interface HasAuthor()
{
    public string $author { get; set; }
}

interface HasContent()
{
    public string $content { get; set; }    
}

interface CanBeLiked()
{
    public function like(): void;
    public function isLiked(): bool;
}

class Post implements HasAuthor, HasContent, CanBeLiked
{
    public string $author = '...';
    public string $content = '...';
    public function like(): void { ... }
    public function isLiked(): bool { ... }
}

This principle is more achievable in PHP now that we have Intersection Types. For example:

class Thread
{
    /** @var HasAuthor&HasContent&CanBeLiked[] */
    private array $replies = [];

    public function __construct(private HasAuthor&HasContent $subject)
    {
    }

    public function display()
    {
        echo $this->subject->content . '<br>';
        echo 'Author: ' . $this->subject->author . '<br><br>';
        foreach ($this->replies as $reply) {
            echo $reply->content . '<br>';
            echo 'Author: ' . $reply->author . '<br><br>';
            if ($reply->isLiked()) {
                echo '(Someone has liked this reply!)<br><br>';
            }
        }
    }
}

The Thread class doesn't know anything about a Post class; it just knows that its $subject property must implement both HasAuthor and HasContent interfaces and that $replies must be an array of elements implementing (via static analysis), HasAuthor, HasContent, and CanBeLiked. These could be instances of Post, Comment, Letter, Car, Dog... anything really. Thread doesn't care.

It's easy to go overboard with this approach, creating dozens of tiny interfaces with massive intersectional types. As with much in coding, try to strike a balance between readability and loose coupling!

Hello Jeffrey! I'm from Central Asia, Tajikistan! And I am excited 😊🙌🏻

I’m thrilled by how attentively and with such joy Kermit listened to today’s lesson. <3 I recently watched the previous version of this series, and I have to admit—you did a damn good job refreshing this material. Keep it up!

@Kobzon Thank you!

Best php course I've ever seen. Thanks @jeffreyway !

Thank you for this wanderfull course!

Thank you very much for the course, once again. It was amazingly interesting!

@JeffreyWay you're the best mentor! Thanks a lot for this course!

@junaidqadir There's an old saying in chinese. "He who by reviewing the old can gain knowledge of the new and is fit to be a teacher. "

It's fun to see that we span the globe. :)

Hello Jeffrey! I'm from Central Asia, Tajikistan! And I am excited 😊🙌🏻

Chattanooga, Tennessee in the US here. :)