小弟我的第一个ajax程序没有输出？

var xmlhttp;
if (window.XMLHttpRequest)
  {// code for Firefox, Opera, IE7, etc.
  xmlhttp=new XMLHttpRequest();
  }
else if (window.ActiveXObject)
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }

function ajax(i){
    var url = "/chat.php";        
    xmlhttp.open("GET",url,true);
    xmlhttp.onreadystatechange = other();
    xmlhttp.send(null);
}

function other(){
    
        document.getElementById("php").innerHTML=xmlhttp.responseText;

}