PHP with AJAX Problem

[bharanidharanit]

Hi,
In my application i am checking for the existing username with AJAX, I used this coding, when the user submits the form it starts checking. It will display message if the username already exists, Now if the username already exists means the form must not return, it must show the message again when the user again submits the form. Any ideas??
Javascript

function exists_username(field)
{
	field = field.value;
	var url = "functions/main.php?ud="+ field;
	if(XMLHttpRequestObject) 
    {
	  var obj = document.getElementById("v1"); 
      XMLHttpRequestObject.open("GET", url); 
      XMLHttpRequestObject.onreadystatechange = function() 
      { 
        if (XMLHttpRequestObject.readyState == 4 && 
          XMLHttpRequestObject.status == 200) 
          { 
			obj.innerHTML = XMLHttpRequestObject.responseText;
          } 
      } 
      XMLHttpRequestObject.send(null); 
    }
}

PHP Function

function checkusername($ud)
{
	require_once('dbconnect.php');
	$query = "SELECT username
			  FROM users
			  WHERE username='$ud';";
	$result = mysqli_query($dbconn,$query);
	$userdata = mysqli_fetch_array($result,MYSQL_ASSOC);
	if($ud = $userdata['username'])
	{echo "Username Already Exists";}	
}