book_client.php

<?php
/**
* Single Responsiblity Principle client code
*/

// create a book instance, book has no knowledge about rendering
$book = new Book();
$book->setTitle("Some Title");
$book->setAuthor("Some Author");

// now render it in plain text, specialized in Plain text only!
$plain_renderer = new PlainTextRenderer();
echo $plain_renderer->render($book->getCurrentPage());

// and now render in html, specialized in HTML only!
$html_renderer = new HTMLRenderer();
echo $html_renderer->render($book->getCurrentPage());