gistfile1.php

<?php
class ContentAppender {
	private $message ='';
	public function __construct( $message ) {
		$this->message = $message;
		add_filter( 'the_content', array( $this, 'add_content' );
	}
	
	public function add_content( $content) {
		return  $content.$this->message;
	}
}
 
// print a message in the footer
$contentappender = new ContentAppender( 'This appears at the end of the content' );