http://php.net/manual/en/filter.filters.sanitize.php

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);

$controller = new AddBookController($bookStore);

echo $controller->addBook();

AddBookController.php

require_once("BookStore.php");
require_once("BookStoreView.php");
require_once("BookView.php");

/**
* AddBookController handles the use case Add book
* Use-Case starts when user wants to add a book
* User presents Book title, author and ISBN number to system
* The system confirmes that the book has been added
* and shows all books in the store.
*/
class AddBookController {
	private $bookStoreView;
	private $bookView;

	private $bookStore;


	public function __construct(BookStore $bookStore) {
		$this->bookStore = $bookStore;
		$this->bookStoreView = new BookStoreView($this->bookStore);
		$this->bookView = new BookView();
	}
	/**
	* @return String HTML
	*/
	public function addBook() {
		$this->handleInput();

		//Combine Output
		return $this->bookStoreView->getHTML() . $this->bookView->getHTML();
	}

	private function handleInput() {
		if ($this->bookView->userWantsToAddBook()) {
			try {
				$book = $this->bookView->getNewBook();

				if ($this->bookStore->addBook( $book ) == false) {
					$this->bookView->showThisBookExists();
				} else {
					$this->bookView->bookAddedSuccess();
				}
			} catch (Exception $exception) {
				
			}
			
		}
	}
}

BookView.php

class BookView {
	private static $AuthorName = "BookView::AuthorName";
	private static $TitleName = "BookView::TitleName";
	private static $ISBNName = "BookView::ISBNName";
	private static $addBook = "addBook";

	private $message = "";

	/**
	* @return boolean true if user wants to add a book
	*/
	public function userWantsToAddBook() {
		if (isset($_GET[self::$addBook])) 
			return true;
		return false;
	}

	/**
	* @return String HTML
	*/
	public function getHTML() {
		//previous input
		
		$author = $this->getCleanInput(self::$AuthorName);
		$title = $this->getCleanInput(self::$TitleName);
		$isbn = $this->getCleanInput(self::$ISBNName);
		
		return "

			<div>
				<form action='?" . self::$addBook . "' method='post' enctype='multipart/form-data'>
				<fieldset>
					<legend>Add a new book</legend>
					<label for='AuthorID' >Author :</label>
					<input type='text' name='". self::$AuthorName ."' id='AuthorID' value='$author' />
					<label for='TitleID' >Title :</label>
					<input type='text' name='". self::$TitleName ."' id='TitleID' value='$title' />
					<label for='ISBNID' >ISBN :</label>
					<input type='text' name='". self::$ISBNName ."' id='ISBNID' value='$isbn' />
					<input type='submit' value='Add Book'/>
					$this->message
				</fieldset>

				</form>
			</div>";
	}

	/**
	* @return Book 
	* @throws Exception if something is wrong
	*/
	public function getNewBook() {
		$author = $this->getCleanInput(self::$AuthorName);
		$title = $this->getCleanInput(self::$TitleName);
		$isbn = $this->getCleanInput(self::$ISBNName);

		try {
			return new Book($author, $title, $isbn);
		} catch (Exception $e) {
			$this->message = "All fields need to be set";
			throw $e;
		}
	}

	public function bookAddedSuccess() {
		$this->message = "The book was successfully added...";
	}

	public function showThisBookExists() {
		$this->message = "The book already exists in library...";
	}

	

	/**
	* @param String input
	* @return String input - tags - trim
	* @throws Exception if something is wrong or input does not exist
	*/
	private function getCleanInput($inputName) {
		if (isset($_POST[$inputName]) == false) {
			return "";
		}
		
		return $this->sanitize($_POST[$inputName]);
	}

	/**
	* @param String input
	* @return String input - tags - trim
	*/
	private function sanitize($input) {
		$temp = trim($input);
		return filter_var($temp, FILTER_SANITIZE_STRING, FILTER_FLAG_STRIP_LOW);
	}
}

BookStore

require_once("Book.php");

/**
* BookStore Contains a record of which books are available
* in a BookStore
*/
class BookStore {
	/**
	* @var array of Book objects
	*/
	private $books = array();

	/**
	* @param Book book The book to be added
	* @return boolean Return true if book was added, 
	*                        false if book already was in store...
	*/
	public function addBook(Book $book) {
		if ($this->isBookInStore($book))
			return false;

		//Add book to the end of the array
		$this->books[] = $book;

		return true;
	}

	/**
	* @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;
	}



	/**
	* @param Book book, book to be compared to Store
	* @return boolean true if book exists in store
	*/
	private function isBookInStore(Book $book) {
		//isBookin store
		foreach ($this->books as $inStoreBook) {
			if ($inStoreBook->isSame($book)) {
				return true;
			}
		}
		return false;
	}
}

Book.php

/**
* An example class with member variables
* http://www.phpdoc.org/
*/
class Book {

	/**
	* @var String
	*/
	public $author;

	/**
	* @var String
	*/
	public $title;

	/**
	* @var String
	*/
	public $isbn;

	/**
	* @param String author , Example "J.K. Rowling"
	* @param String title , Example "Harry Potter and the Philosopher's Stone"
	* @param String isbn , Example "9788478888566"
	* @throws Exception if a book could not be constructed
	*/
	public function __construct($author, $title, $isbn) {
		if ($author == "")
			throw new Exception("Cannot create book without author");
		if ($title == "")
			throw new Exception("Cannot create book without title");
		if ($isbn == "")
			throw new Exception("Cannot create book without isbn");

		$this->author = $author;
		$this->title = $title;
		$this->isbn = $isbn;

	}

	/**
	* @param Book other book to compare to
	* @return boolean return true if the books are the same
	*/
	public function isSame(Book $other) {
		if($this->author != $other->author) { 
			return false;
		}

		if($this->title != $other->title) { 
			return false;
		}

		if($this->isbn != $other->isbn) { 
			return false;
		}

		return true;
	}

	/**
	* @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;
	}
}

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";
	}

}