php84/Books.php

<?php
interface HasAuthors {
    public string $credits { get; }
    public Author $mainAuthor { get; set; }
}

class Author {
    public function __construct(public private(set) string $name) {}
}

class Book implements HasAuthors {
    public string $credits {
        get => "Authors: David Smith\n";
    }

    public Author $mainAuthor {
        get => new Author("Smith");
        set => throw new Exception("Main author cannot be changed.");
    }
}

$book = new Book();
echo $book->credits;

$author = $book->mainAuthor;
echo $author->name;