Ajax Example
Screen shot
HTML Page

<html>
<head>
<script type="text/javascript" src="ajax.js">
</script>
<style type="text/css">
<!--
.style1 {
color:#FF3300;

}
-->
</style>
</head>
<body>
<table width="309" height="211" border="0" cellpadding="0" cellspacing="0">
<tr>
<td height="39" colspan="2" align="center">Create User Account </td>
</tr>
<tr>
<td>User ID</td>
<td><input name="userid" type="text" onKeyUp="display(this.value)">
<div class="style1" id="showuser"></div></td>
</tr>
<tr>
<td>User Password</td>
<td><input name="userpassword" type="text"></td>
</tr>
<tr>
<td>User Name </td>
<td><input name="username" type="text" id="username"></td>
</tr>
<tr>
<td>Email</td>
<td><input name="email" type="text" id="email"></td>
</tr>
<tr>
<td>&nbsp;</td>
<td><input name="btnsubmit" type="submit" value="Submit"></td>
</tr>
</table>
</body>
</html>

Javascript Page (save as ajax.js)

var XMLHttpRequestObject=false;

function display(user)
{
if(window.XMLHttpRequest)
{
XMLHttpRequestObject=new XMLHttpRequest();
}
else if(window.ActiveXObject)
{
XMLHttpRequestObject=new ActiveXObject("Microsoft.XMLHTTP");
}

XMLHttpRequestObject.onreadystatechange=function()
{
if (XMLHttpRequestObject.readyState==4 && XMLHttpRequestObject.status==200)
{
document.getElementById("showuser").innerHTML=XMLHttpRequestObject.responseText;
}
}
XMLHttpRequestObject.open("GET","users.php?id="+user,true);
XMLHttpRequestObject.send();
}

PHP Page(save as users.php)

<?php
$con=mysql_connect('localhost','root','') or die('Mysql not connected');
mysql_select_db('users',$con) or die('DataBase not connected');

$id=$_GET["id"];
$query="select * from user_login WHERE user_id = '".$id."'";
$result = mysql_query($query);
$num=mysql_num_rows($result);
if($num>0)
echo "User already Exist";
?>

DataBase

--
-- Database: `users`
--

-- --------------------------------------------------------

--
-- Table structure for table `user_login`
--

CREATE TABLE IF NOT EXISTS `user_login` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` varchar(100) NOT NULL,
`user_password` varchar(100) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=3 ;

--
-- Dumping data for table `user_login`
--


INSERT INTO `user_login` (`id`, `user_id`, `user_password`) VALUES
(1, 'shishir012010', 'bcamca'),
(2, 'ravi2010', '9953891928');