XMLHttpRequest get post

https://www.pocketdigi.com/20110425/260.html

var xmlHttp;
function createXmlHttpRequest(){
    if (window.ActiveXObject) {
        xmlHttp = new window.ActiveXObject("Microsoft.XMLHTTP");
    }
    else 
        if (window.XMLHttpRequest) {
            xmlHttp = new XMLHttpRequest();
        }
}
createXmlHttpRequest();
function GET(){
    xmlHttp.open("GET", "3.php", true);
    xmlHttp.onreadystatechange = function(){
        if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
            alert(xmlHttp.responseText);
        }
    }
    xmlHttp.send(null);
}
function POST(){
    xmlHttp.open("POST","3.php");
    alert("POST");
    xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
    xmlHttp.onreadystatechange = function(){
        if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
            alert(xmlHttp.responseText);
        }
    }
    xmlHttp.send("name=zhang");
}