Remote check to see if user exists?

Hi I am using Jquery to validate a form and I need to be able to check to see if the user exists by doing a callback to a custom php file check-user.php
I have included includes/user.php but I seem to be missing something.
I have searched for a while and come up with nothing what else do I need to include before includes/user.php to make wordpress work with this?

<?php
include("config.php");
include( $path . 'wp-admin/includes/user.php' );
$username = $_POST['user_login'];
function username_exists( $username ) {
	if ( $user = get_user_by( 'login', $username ) ) {
		return $user->ID;
	}
	return false;
}
$ckuser  = username_exists( $username );
echo $ckuser;
?>