php中使用ajax时ResponseText返回undefined
<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->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!!!