I want the AJAX file loader, for example, to load a Gmail file with a progress bar in PHP

You can dynamically update the JQuery Progress Bar plugin and achieve the same look.

<div id="progressbar"></div>
<script>
  $(document).ready(function() {
    $("#progressbar").progressbar({ value: 1 });
  });
</script>

Then all you have to do is reference the progress bar and its value based on your actual progress:

$( ".selector" ).progressbar( "option", "value", <YOURVALUE>);