var ajax = new Array();

function getSubCategoryList(sel)
{
   var category = sel.options[sel.selectedIndex].value;
   //alert(category);
   document.getElementById('sub_category').options.length = 0;   // Empty city select box
   if(category.length>0){
      var index = ajax.length;
      ajax[index] = new sack();

      ajax[index].requestFile = 'getSubCategories.php?category='+category;   // Specifying which file to get
      ajax[index].onCompletion = function(){ createSubCategories(index) };   // Specify function that will be executed after file has been found
      ajax[index].runAJAX();      // Execute AJAX function
   }
}

function createSubCategories(index)
{
   var obj = document.getElementById('sub_category');
   eval(ajax[index].response);   // Executing the response from Ajax as Javascript code
}