<?php
// Entity
class Book 
{
    private $id;
    private $title;

    public function __construct($id, $title)
    {
        $this->id = $id;
        $this->title = $title;
    }

    ... getters and setters methods ...
}

// DAO (Data access object)
class BookDao 
{
    public function findBookByName($name)
    {
        $db = Database::getInstance();
        $row = $db->select('book')->where('name = ?', $name);
        return new Book($row['id'], ['title']);
    }
}

// Registry
class Registry
{
    private static $objects = array();

    public static function get($index){}
    public static function set($index, $value){}
    public static function isRegistered($index){}
}

$dao = new BookDao();
Registry::set('BookDao', $dao);

...

$dao = Registry::get('BookDao');
$book1 = $dao->findBookByName('Example 1');
$book2 = $dao->findBookByName('Example 2');