创建兼容于多浏览器支持的XMLHTTPRequest

try{
if( window.ActiveXObject ){
for( var i = 5; i; i-- ){
try{
if( i == 2 ){
xmlhttp_request = new ActiveXObject( "Microsoft.XMLHTTP" );
}else{
xmlhttp_request = new ActiveXObject( "Msxml2.XMLHTTP." + i + ".0" );
}

//xmlhttp_request.setRequestHeader方法必须在open方法之后被调用！！
//xmlhttp_request.setRequestHeader("Content-Type","text/xml");
//xmlhttp_request.setRequestHeader("Content-Type","gb2312");
break;}
catch(e){
xmlhttp_request = false;
}
}
}else if( window.XMLHttpRequest ){
xmlhttp_request = new XMLHttpRequest();
if (xmlhttp_request.overrideMimeType) {
xmlhttp_request.overrideMimeType(''text/xml'');
}
}
}catch(e){
xmlhttp_request = false;
}
文章出处：飞诺网(www.firnow.com):http://dev.firnow.com/course/1_web/webjs/2008515/116676.html