AJAX 用户唯一性验证实现代码

!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 send_request(url,data){
createXMLHttpRequest();
xmlHttp.open("POST",url,true);
xmlHttp.onreadystatechange = check_lll;
xmlHttp.setRequestHeader("CONTENT-TYPE", "application/x-www-form-urlencoded");
xmlHttp.send("username=" + data);
}
function check_lll(){
if(xmlHttp.readyState == 4){
if(xmlHttp.status == 200){
alert(xmlHttp.responseText);
}
}
}
function check_username(){
var f = document.form1;
var username = f.username.value;
if(username == ""){
alert("NULL");
return false;
}
else{
send_request("check_it.php",username);
}
}
/script>
body>
form id="form1" name="form1" method="post" action="">
p>nbsp;/p>p>
姓名： input type="text" name="username" />
/p>
input type="button" value="check it" onclick="check_username()" />
p>nbsp;/p>
p>nbsp; /p>
/form>
/body>
/html>

?php
$username = $_POST["username"];
$conn = mysql_connect("localhost:3306","root","123");
mysql_select_db("my",$conn);
$sql = "select * from username where username = '$username'";
$result = mysql_query($sql,$conn);
$num = mysql_fetch_array($result);
if($num > 0){
printf("can't use");
}
else{
printf("It can use");
}