What are some practical uses for callback functions?

I'm trying to figure out how callbacks can come in useful in PHP. I found this tutorial:

http://www.phpriot.com/articles/php-callbacks

which gives an example of a script which downloads an RSS feed every 2 seconds (its put on sleep(2)) and rather than wait until the script has finished (which could take a long time if theres a lot of RSS feeds to download) to output the results, it prints the results each time it downloads a single RSS feed. This sounds useful, but I tested it out:

<?php
    // this function simulates downloading data from a site
    function downloadFromSites($sites, $callback = null)
    {
        $ret = array();
 
        foreach ($sites as $site) {
            sleep(5);
 
	    $xmlDoc = new DOMDocument();
            $data = $xmlDoc->load($xml);
            $sxml = simplexml_import_dom($data);
            

            // check if the callback is valid
            if (is_callable($callback)) {
                // callback is valid - call it with given arguments
                call_user_func($callback, $site, $data);
            }
 
            // write the data for this site to return array
            $ret[] = array(
                'site' => $site,
                'data' => $data
            );
        }
 
        return $ret;
    }
 
    // define a fictional class used for the callback
    class MyClass
    {
        // this is the callback method
        public static function downloadComplete($site, $data)
        {
            echo sprintf("Finished downloading from %s\n", $site);
            echo '<br>';
        }
    }
 
    // imaginary list of sites to download from
    $sites = array(
        'http://news.google.com/news?ned=us&topic=h&output=rss',
        'http://rss.msnbc.msn.com/id/3032091/device/rss/rss.xml'
        // more sites...
    );
 
    // start downloading and store the return data
    downloadFromSites($sites, array('MyClass', 'downloadComplete'));
 
    // we don't need to loop over the return data
    // now since the callback handles that instead
?>

and it doesn't work. It just prints the results when the script has finished running. What am I doing wrong here?