var url = "includes/fetch_subcat.php"; 
 
function fetch_subcat() {
  new Ajax.Request(url, {
      method: 'get',
      parameters: {cat_id: document.getElementById('post_category_id').value },
      onSuccess: process,
      onFailure: function() { 
      alert("There was an error with the connection"); 
    }
  });
}

function process(transport) {
  var response = transport.responseText;
  var div_obj = document.getElementById('subcatdiv');
  div_obj.innerHTML = response;  
}