Testfall

//TestCase
$bookUnlimited = new Book("Daniel Toll", "PHP Unlimited", "123456");
$bookUnlimited2 = new Book("Daniel Toll", "PHP Unlimited II", "123457");
$bookDuck = new Book("Arne Anka", "Duck Tales", "7654321");

$bookStore = new BookStore();

$bookStore->AddBook($bookUnlimited);
$bookStore->AddBook($bookUnlimited2);
$bookStore->AddBook($bookDuck);

$bookStoreView = new BookStoreView($bookStore);

echo $bookStoreView->getHTML();

BookStoreView.php

require_once("BookStore.php");

/**
* BookStoreView visualizes a BookStore in HTML
*/
class BookStoreView {

	/**
	* @var BookStore
	*/
	private $bookStore;

	/**
	* @param BookStore
	*/
	public function __construct(BookStore $store) {
		$this->bookStore = $store;
	}

	/**
	* @return String (HTML)
	*/
	public function getHTML() {
		$returnValue = "<h2>Book Store</h2>";

		$sortedBooks = $this->bookStore->getByAuthorsSorted();

		$returnValue .= "<ol>";
		
		foreach ($sortedBooks as $book) {
			$returnValue .= "<li>" . $this->getBookHTML($book) . " </li>";
		}
		$returnValue .= "</ol>";


		return $returnValue;
	}
	/**
	* @param Book
	* @return String (HTML)
	*/
	private function getBookHTML(Book $book) {
		return "
				<div>
					<h3>$book->title</h3>
					<p>Author: $book->author</p>
					<p>ISBN: $book->isbn</p>
				</div> n";
	}

}

BookStore

       /**
	* @return array of Book objects sorted by author
	*/
	public function getByAuthorsSorted() {
		//http://se1.php.net/usort
		$sortOk = usort($this->books, array("Book", "compareByAuthor") );

		assert($sortOk);

		return $this->books;
	}

Book

       /**
	* @param Book a
	* @param Book b
	* @return int return 1 if  a > b
	*             return 0 if  a = b
	* 			  return -1 if a < b
	*/
	public static function compareByAuthor($a, $b) {

		//http://se1.php.net/usort# Example #3
 		$al = strtolower($a->author);
        $bl = strtolower($b->author);
        if ($al == $bl) {
            return 0;
        }
        return ($al > $bl) ? +1 : -1;
	}