Check Username Availability Issue

I am just adding a new feature to my Joomla TPJOBS component although it's not a good/complete/active component but just I'm adding a user name Availability checker but its not wor

Solution 1:

<?php
defined('_JEXEC') or die('Restricted access');


$user = strip_tags(trim($_REQUEST['username']));

if(strlen($user) <= 0)
{
  echo json_encode(array('code'  =>  -1,
  'result'  =>  'Invalid username, please try again.'
  ));
  die;
}

// Query database to check if the username is available
$query = "Select a.username FROM #__users where username = '$user' ";
// Execute the above query using your own script and if it return you the
// result (row) we should return negative, else a success message.

$db =& JFactory::getDBO();

$result = $db->setQuery($query);

$available = mysql_num_rows($result);

if($available)
{
  echo json_encode(array('code'  =>  1,
  'result'  =>  "Success,username $user is still available"
  ));
  die;
}
else
{
  echo  json_encode(array('code'  =>  0,
  'result'  =>  "Sorry but username $user is already taken."
  ));
  die;
}
die;
?>