function PostBack(url, parameters, response_container_id, loading_msg, success_callback) {

      http_request = false;
	  var returned_response_check=1;
      if (window.XMLHttpRequest) { // Mozilla, Safari,...
         http_request = new XMLHttpRequest();
         if (http_request.overrideMimeType) {
            http_request.overrideMimeType('text/html');
         }
      } else if (window.ActiveXObject) { // IE
         try {
            http_request = new ActiveXObject("Msxml2.XMLHTTP");
         } catch (e) {
            try {
               http_request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {}
         }
      }
      if (!http_request) {
         alert('Your browser does not support xml http request =]');
         return false;
      }
      
      http_request.onreadystatechange = function() {
		  
		  switch(http_request.readyState) {
			  
			case 1: //sending
				document.getElementById(response_container_id).innerHTML = loading_msg;
				break;
			case 4: //success
				if (http_request.status == 200) {
			
					if( http_request.responseText != returned_response_check )
						document.getElementById(response_container_id).innerHTML = http_request.responseText;
					else
						eval(success_callback);
				}
				else {
					document.getElementById(response_container_id).innerHTML = 'Error: '+http_request.status+': '+http_request.statusText;	
				}
				break;
		  } //switch
	  } //onreadystatechange function
      http_request.open('POST', url, true);
      http_request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
      http_request.setRequestHeader("Content-length", parameters.length);
      http_request.setRequestHeader("Connection", "close");
      http_request.send(parameters);
   }
   
   function checkUsername(url) {
		var param = "check_mem_username="+document.getElementById('check_mem_username').value;
		//var loading = '<img src="members/refresh.gif" />';
		var loading = '<span style="font-size: 11px;">Loading response..</span>';
		var call_back=null;
		if( document.getElementById('span_err_username') ) {
			document.getElementById('span_err_username').innerHTML='';
		}
		PostBack(url, param, 'check_username_resp', loading, call_back);
   }