Progress bar for a lengthy PHP process

If you ask an average web-developer if it's possible to make a running progress bar on the web-page while it's loading(!), most of them will say "no". This is understandable as practically all web-pages do not exhibit such progress bars and are in a sense "static" - the whole web-site is dynamically generated from a database, but each page is static except for AJAX changes due to user actions. Here I will show how a lengthy PHP process can show a running progress bar while the page is loading. Of course, it doesn't make sense for pages loading within a second or three, but those import/export/dump/analyze/recode/generate scripts which takes ages.

The most trivial approach is to echo '.'; flush(); - it will display a dot for every loop iteration. Test this:

for ($i = 0; $i < 100; $i++) {
    echo '.';
    flush();
    usleep(1000000*0.1); // total time 10 sec.
}

The output should be an increasing amount of dots progressively appearing on the screen during 10 seconds. We need to use flush(); in order to send the PHP output buffer out to the browser. Alternatively we could set ini_set('output_buffering', 0);
There is another trick we need to use sometimes, on some servers flush(); doesn't work alone - either Apache or browsers (particularly IE) are buffering the incoming HTML data as well. So I add echo str_repeat(' ', ini_get('output_buffering')); before flush(); to make sure the buffer is completely filled and being flushed automatically.
But we can improve the aesthetics of the output using some JavaScript. Remember, in the old days we were using write('Some HTML'); to generate some dynamic HTML by JavaScript while the page is loading(!)? So JavaScript can be executed immediately and not only onDOMReady.
So we build a progress bar using a container <div> and inside <div> of a different background color which will  change it's width on the commands from JavaScript while the page in loading(!). Everything is encapsulated into a single class.
Usage example:
<?php

require_once 'nadlib/class.ProgressBar.php';

echo 'Starting&hellip;<br />';

$p = new ProgressBar();
echo '<div style="width: 300px;">';
$p->render();
echo '</div>';
for ($i = 0; $i < ($size = 100); $i++) {
$p->setProgressBarProgress($i*100/$size);
usleep(1000000*0.1);
}
$p->setProgressBarProgress(100);

echo 'Done.<br />';

Download class: http://pastebin.com/KSxjC01r
I hope using this technique will make your waiting more pleasant. You can even spot potential problems if the progress bar stops for unusually long time or is going slower and slower. I once improved the performance by adding a database index after watching the progress bar behavior.
Disclaimer: this idea is inspired by TYPO3 Extension Manager.