AJAX 实时读取输入文本(php)

!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
html xmlns="http://www.w3.org/1999/xhtml">
head>
meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
title>无标题文档/title>
/head>
script language="javascript">
var xmlHttp;

function createXMLHttpRequest(){
if(window.ActiveXObject){
xmlHttp = new ActiveXObject("microsoft.XMLHTTP");
}
else if(window.XMLHttpRequest){
xmlHttp = new XMLHttpRequest();
}

}

function sendRequest(){

createXMLHttpRequest();

var name = document.getElementById("name").value;
url = "String_check.php?page="+name;
xmlHttp.onreadystatechange = callback;
xmlHttp.open('GET',url,true);
xmlHttp.send(null);
}

function callback(){
if(xmlHttp.readyState == 4){
if(xmlHttp.status == 200){
document.getElementById("show").innerHTML = "您输入的字符串为："+ xmlHttp.responseText;

}
}
}

/script>
body>
p>AJAX test;/p>
p>br/>
input type="text" id="name" onkeyup="sendRequest();" />
br/>
span id="show">/span>
/p>
/body>
/html>

?php
header('Content-type: text/html;charset=GB2312');
$ename = $_GET["page"];
if ($ename == "") {
$ename = "请输入：";
}
echo ($ename);
exit(0);
?>