Th JavaScript code for this will be similar to: (You will need to modify this)
<script type="text/javascript">
function createRequestObject() {
var ro;
var browser = navigator.appName;
if(browser == "Microsoft Internet Explorer") {
ro = new ActiveXObject("Microsoft.XMLHTTP");
}else {
ro = new XMLHttpRequest();
}
return ro;
}
var http = createRequestObject();
function sndReq(action) {
if(action.length < 3) {
if(action.length == 0) {
document.getElementById('usernamecheck').innerHTML = "<br />";
return false;
}
document.getElementById('usernamecheck').innerHTML = "Username too short";
return false;
}else {
http.open('get', 'rpc.php?action='+action);
http.onreadystatechange = handleResponse;
http.send(null);
}
}
function handleResponse() {
if(http.readyState == 4) {
var update = http.responseText;
if(update!="This name is Avaliable!") {
document.getElementById('usernamecheck').innerHTML = "This name is unavaliable, please try again";
}else {
document.getElementById('usernamecheck').innerHTML = "<a href='Javascript:EnableDisable()'>"+update+" Click to change</a>";
}
}
}
</script>
HTML for the username box:
<input type="text" name="un" onchange="sndReq(this.value)">
And rpc.php
<?
//connect to db first
$user=mysql_query("SELECT * FROM user_table WHERE un = '".$_REQUEST['action']."'")or die(mysql_error());
$countrows=mysql_num_rows($user);
if($countrows > 0) {
echo "This name is Unavaliable";
}
else {
echo"This name is Avaliable!";
}
?>
Note, the PHP code if not good, there is no checking for bad data so it is not secure, just an example
Unsolved 
