php中运用ajax时ResponseText返回undefined

var URL, ReturnHtml;
var objXMLHTTP = new GetXmlHTTP();
alert(objXMLHTTP); //FF下创建对象XMLHttpRequest成功
URL = "getdata.php?username="+ document.getElementById("username").value;
objXMLHTTP.open("GET", URL, false);
objXMLHTTP.send(null);
ReturnHtml = objXMLHTTP.ResponseText;
alert(ReturnHtml); //FF下总是返回undefined，十分崩溃!!!