PHP: Hide Progress Bar After Page Fully Loaded

How do you show a progress bar when something has partially loaded, and get rid of it on the same page when it is done loading?

<?php
echo "<div id='a'><center>Processing Data<br>";
echo "<img src='http://www2.bulkregister.com/images/progressbar.gif'">;
echo "</center></div>";
flush();
sleep(5);//do real work or just sleep (for testing)
echo "Afterwords: <br>";
$js="'document.getElementById(\"a\").style.display=\"none\"'";
echo "<img src='http://www.cwts.nl/ed/buttons/completed.gif' 
onload='$js'>";
?>