function(data) //return information back from jQuery's get request
{
$('#progress_container').fadeIn(100); //fade in progress bar
$('#progress_bar').width(data +"%"); //set width of progress bar based on the $status value (set at the top of this page)
$('#progress_completed').html(parseInt(data) +"%"); //display the % completed within the progress bar
}
)},500);

Nella function fai console.log(data) e vedi cosa effettivamente ti viene tornato dalla richiesta asincrona