xmlhttp.responseText 取值不到，回调函数最主要

xmlhttp.responseText 取值不到，回调函数最重要
function getDataByAjax(requestType,url){
var xmlhttp = createXMLHTTP();
if(window.XMLHttpRequest){
  xmlhttp=new window.XMLHttpRequest();
        //IE7.0,IE8.0,FireFox,Mozilla,Safari,Opera
        xmlhttp = new XMLHttpRequest();
        if(xmlhttp.overrideMineType){
            xmlhttp.overrideMineType("text/xml");
        }
}else if(window.ActiveXObject){
xmlhttp=new ActiveXObject(getXmlHttpPrefix()+".XmlHttp");
}
    var xmlHead="<?xml version="1.0" encoding="GBK"?>";
    if(url.indexOf("?")>0)
       url+="&randomCode="+Math.round(Math.random()*9999999);
    else
       url+="?randomCode="+Math.round(Math.random()*9999999);
    xmlhttp.open(requestType, url, false);  //同步方式进行
    xmlhttp.send(null);
    if(xmlhttp.readyState == 4){
        if(xmlhttp.status == 200){
                returnHtml=xmlhttp.responseText;
        }else{
                alert("您所请求的页面有误...");
        }
}
return returnHtml;
}