PHP: Implementing the SeekableIterator interface

class BookShelf implements ArrayAccess, Countable, SeekableIterator {
    protected $_books = array(...);
    protected $_magazines = array(...);

    /**** CLASS CONTENT HERE ****/
}

$shelf = new BookShelf();

// Loops through both arrays, first books (if any) and then magazines (if any)
foreach($shelf as $item) {
    echo $item;
}

count($this->_books);

$this->_books[$index];

$this->_magazines[$index-count($this->_books)]