รบกวนถามเรื่องการ selector ของ jQuery จากค่าตัวแปรของ php ครับ และส่งไปทาง url

<script type="text/javascript">
$(document).ready(function(){

function last_msg_funtion() 
{ 
var productCategory = "<?php echo (isset($productCategory) ? $productCategory : '' ;?>";
var ID=$(".message_box:last").attr("id");
$('div#last_msg_loader').html('<span>Loding</span>');
$.post("/e_com/scrolls/index?action=get&last_msg_id="+ID,

function(data){
if (data != "") {
$(".message_box:last").after(data);	
}
$('div#last_msg_loader').empty();
});
}; 

$(window).scroll(function(){
if ($(window).scrollTop() == $(document).height() - $(window).height()){
last_msg_funtion();
}
}); 

});
</script>

<form id='form'>
    <input type="text" name="username" id="username" size="30" value="<?php echo $productCategory;?>">
    </form>



<script type="text/javascript">
	$(document).ready(function(){
            
                          
                
                
			
		function last_msg_funtion() 
		{ 
		var productCategory =  $.trim($('#form input[name=username]').val());
                
                var ID=$(".message_box:last").attr("id");

           
			$('div#last_msg_loader').html('<span>Loding</span>');
			$.post("/e_com/scrolls/index?action=get&last_msg_id="+ID+"&productCategory="+productCategory,

			
			function(data){
				if (data != "") {
				$(".message_box:last").after(data);			
				}
				$('div#last_msg_loader').empty();
			});
		};  
		
		$(window).scroll(function(){
			if  ($(window).scrollTop() == $(document).height() - $(window).height()){
			   last_msg_funtion();
			}
		}); 
		
	});
	</script>