保持jQuery可折叠菜单栏的扩展

Menu collapses when link is clicked because it loads a new page. Is there anyway I can keep the menu expanded using php variable?

<ul class="<? echo $page_id; ?>">

<script>
$(document).ready(function () {

 $('.page_id').slideDown(); // this is where I would like to use variable!

 $('#nav > li > a').click(function(){
  if ($(this).attr('class') != 'active'){
    $('#nav li ul').slideUp();
    $(this).next().slideToggle();
    $('#nav li a').removeClass('active');
    $(this).addClass('active');
    }
  });
});
</script>

Thanks for teaching!

on page load open it:

<script>
$(document).ready(function () {
  //slide down on page load
  $('#nav li ul').slideDown();

 $('#nav > li > a').click(function(){
  if ($(this).attr('class') != 'active'){
    $('#nav li ul').slideUp();
    $(this).next().slideToggle();
    $('#nav li a').removeClass('active');
    $(this).addClass('active');
    }
  });
});
</script>