I am wondering how to implement PubSubHubBub in a PHP site.I don't understand it.Can you explain me? I don't get the idea. The publisher notifies the subscriber and the subscriber - my site?

    <?php

// simple example for the PHP pubsubhubbub Subscriber
// as defined at http://code.google.com/p/pubsubhubbub/
// written by Josh Fraser | joshfraser.com | josh@eventvue.com
// Released under Apache License 2.0

include("PuSHSubscriber.php");

$hub_url = "http://pubsubhubbub.appspot.com";
$callback_url = "url to my site?";

$feed = "feed link";

// create a new subscriber
$s = new Subscriber($hub_url, $callback_url);

// subscribe to a feed
$s->subscribe($feed);
// unsubscribe from a feed
//$s->unsubscribe($feed);

?>

Ya $ hub_url ben hubumu göndermek gerekir?