$_SESSION["PERCENTAGE_DONE"] = $my_calculated_percentage;

//initialize session, set headers etc. before this
echo $_SESSION["PERCENTAGE_DONE"]

<div id="loading" >
<p>
<img src="images/loading.gif">
Please Wait
</p>
</div>

<script type='text/javascript'>
function showLoading() {
$("#loading").show();
}

function hideLoading() {
$("#loading").hide();
}

</script>

<div id='loading' style='display:none;'><img src='loading.gif' /></div>

$('#loading').show();

$('#loading').hide();