Sample class to demonstrate how classes can be used in WordPress.

post-content-appender.php

<?php

/**
 * Class PostContentAppender
 */
class PostContentAppender {

	protected $append = '';

	public function __construct( $append ) {
		$this->append = $append;
		add_action( 'the_content', array( $this, 'append_content' ) );
	}

	public function append_content( $content ) {
		return $content . $this->append;
	}

}

$appender = new PostContentAppender('<p>Posted by Micah Wood</p>');