var http 				= createRequestObject();

function createRequestObject() {
    var ro;
    var browser = navigator.appName;
    if(browser == "Microsoft Internet Explorer"){
        ro = new ActiveXObject("Microsoft.XMLHTTP");
    }else{
        ro = new XMLHttpRequest();
    }
    return ro;
}

function sign_up(root_folder)
{
	http.open('get', root_folder+'sign_up.php?newletter_mail='+document.getElementById('newletter_mail').value);
  http.onreadystatechange = handleSignUpResponse;
  http.send(null);
}

function handleSignUpResponse()
{
	if(http.readyState == 4)
	{
		var response = http.responseText;
		alert(response);
	}
}