<?php namespace Model;

interface PostInterface
{
    public function setId($id);
    public function getId();

    public function setTitle($title);
    public function getTitle();

    public function setContent($content);
    public function getContent();

    public function setComment(CommentInterface $comment);
    public function setComments(array $comments);
    public function getComments();
}

<?php namespace Model;

class Post implements PostInterface
{
    protected $id;
    protected $title;
    protected $content;
    protected $comments = array();

    public function __construct($title, $content, array $comments = array()) {
        $this->setTitle($title);
        $this->setContent($content);
        if (!empty($comments)) {
           $this->setComments($comments); 
        }
    }

    public function setId($id) {
        if ($this->id !== null) {
            throw new BadMethodCallException(
                "The ID for this post has been set already.");
        }
        if (!is_int($id) || $id             throw new InvalidArgumentException(
                "The post ID is invalid.");
        }
        $this->id = $id;
        return $this;
    }

    public function getId() {
        return $this->id;
    }

    public function setTitle($title) {
        if (!is_string($title) 
            || strlen($title)             || strlen($title) > 100) {
            throw new InvalidArgumentException(
                "The post title is invalid.");
        }
        $this->title = htmlspecialchars(trim($title),
            ENT_QUOTES);
        return $this;
    }

    public function getTitle() {
        return $this->title;
    }

    public function setContent($content) {
        if (!is_string($content) || strlen($content)             throw new InvalidArgumentException(
                "The post content is invalid.");
        }
        $this->content = htmlspecialchars(trim($content),
            ENT_QUOTES);
        return $this;
    }

    public function getContent() {
        return $this->content;
    }

    public function setComment(CommentInterface $comment) {
        $this->comments[] = $comment;
        return $this;
    }

    public function setComments(array $comments) {
        foreach ($comments as $comment) {
            $this->setComment($comment);
        }
        return $this;
    }

    public function getComments() {
        return $this->comments;
    }
}

<?php namespace Model;

interface CommentInterface
{
    public function setId($id);
    public function getId();

    public function setContent($content);
    public function getContent();

    public function setAuthor($author);
    public function getAuthor();
}

<?php namespace Model;

class Comment implements CommentInterface
{
    protected $id;
    protected $content;
    protected $author;

    public function __construct($content, $author) {
        $this->setContent($content);
        $this->setAuthor($author);
    }

    public function setId($id) {
        if ($this->id !== null) {
            throw new BadMethodCallException(
                "The ID for this comment has been set already.");
        }
        if (!is_int($id) || $id             throw new InvalidArgumentException(
                "The comment ID is invalid.");
        }
        $this->id = $id;
        return $this;
    }

    public function getId() {
        return $this->id;
    }

    public function setContent($content) {
        if (!is_string($content) || strlen($content)             throw new InvalidArgumentException(
                "The content of the comment is invalid.");
        }
        $this->content = htmlspecialchars(trim($content),
            ENT_QUOTES);
        return $this;
    }

    public function getContent() {
        return $this->content;
    }

    public function setAuthor($author) {
        if (!is_string($author) || strlen($author)             throw new InvalidArgumentException(
                "The author is invalid.");
        }
        $this->author = $author;
        return $this;
    }

    public function getAuthor() {
        return $this->author;
    }
}

<?php use LibraryLoaderAutoloader,   
    ModelPost,
    ModelComment;

require_once __DIR__ . "/Library/Loader/Autoloader.php";
$autoloader = new Autoloader;
$autoloader->register();

$post = new Post(
    "A sample post.",
    "This is the content of the post."
);

$post->setComments(array(
    new Comment(
        "One banal comment for the previous post.",
        "A fictional commenter"),
    new Comment(
        "Yet another banal comment for the previous post.",
        "A fictional commenter")
));

echo $post->getTitle() . " " . $post->getContent() . "<br></br>";

foreach ($post->getComments() as $comment) {
    echo $comment->getContent() . " " . $comment->getAuthor() .
        "<br></br>";
}