ajax的问题

function createXMLHTTP()          //初始化AJAX 对象
{

http_request = false;

if (window.XMLHttpRequest) {
http_request = new XMLHttpRequest();
if (http_request.overrideMimeType) {
http_request.overrideMimeType('text/xml');//修改header
}
} else if (window.ActiveXObject) { // 判定浏览器是 IE
try {
http_request = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
http_request = new ActiveXObject("Microsoft.XMLHTTP");
} catch (e) {}
}
}

if (!http_request) {//初始化xmlhttp组件
alert('Giving up :( Cannot create an XMLHTTP instance');
return false;
}
return http_request;
}

function trim(str)  //去2边空格函数
{
  return str.replace(/(^\s*)|(\s*$)/g,"");
}

function mzrui_login()  //后台登录
{
var check_safe=document.forms["check_safe"];
var http_request=createXMLHTTP();
var str1=escape(check_safe.mzrui_name.value);
var str2=escape(check_safe.mzrui_pass.value);
if(str1 == "" || str2 == "")
{
alert("请输入用户名或密码");
}
else
{
var url=('ajax.php?action=mzrui_login&name='+str1+'&am