gistfile1.php

<?php

class Article extends Content
{
  protected $comments; // a Collection implementation, ArrayCollection or PersistentCollection

  // get the ArrayCollection or PersistentCollection
  public function getComments()
  {
    return $this->comments; 
  }

  // get the CommentCollection to access custom methods.
  public function getCommentCollection()
  {
    return new CommentCollection($this->getComments());
  }
}

class CommentCollection
{
  protected $collection;

  public function __construct(Collection $collection)
  {
    $this->collection = $collection;
  }

  public function getLastComments($limit = null)
  {
    $lastComments= array_reverse($this->collection->toArray());
    if($limit) {
      return array_slice($lastComments,0,$limit);
    }

    return $lastComments;
  }
}

ok, why can't you do:

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

to get rid of getCommentCollection
that way your comments will always behave the same way.

Right. And ideally it should implement the Collection interface.
Maybe something like that:

class CommentCollection extends CollectionWrapper
{
    public function doFancyStuff()
    {
        // Do things with $this->collection
    }
}

class CollectionWrapper implements Collection
{
    protected $collection;

    public function __construct(Collection $collection)
    {
        $this->collection = $collection;
    }

    /* Implement here all Collection methods, forwarding logic to $this->collection */

    public function add($element)
    {
        return $this->collection->add($element);
    }

    /* And more... */
}