how can help me about my code ?

how can help me about my code ?

i have three page : 

index.php : 

  3. <?php 
  4. require_once("includes/funcs.class.php");
  5. $existUsername = new funcs();
  6. if(isset($_POST['username']))
  7. {
  8. $existUsername->existUsername($_POST['username']);
  9. }
  10. ?>
  11. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
  12. <html xmlns="http://www.w3.org/1999/xhtml">
  13. <head>
  14. <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
  15. <title>Untitled Document</title>
  16. <script src="js/jquery-1.9.0.min.js" type="text/javascript"></script>
  17. <script src="js/juser.js" type="text/javascript"></script>
  18. <link rel="stylesheet" href="style.css" />
  19. </head>

  21. <body>
  22. <div class="divclass">
  23. <form action="index.php" method="post" id="login">
  24. Username : <input type="text" name="username" id="username" /><br /><br />
  25. Password : <input type="text" name="password" id="password" /><br /><br />
  26. <input type="submit" value="login" id="submitlogin" /><br /><br />
  27. </form>
  28. </div>
  29. </body>
  30. </html>
funcs.class.php : 

  2. <?php
  3. require_once("config.class.php");
  4. class funcs
  5. {
  6. protected $_dbconnect;
  7. protected $_dbusername;
  8. protected $_dbpassword;
  9. protected $_dbemail;
  10. //User's ... Functions ...
  11. function __construct()
  12. {
  13. return config::getInstance();
  14. }
  15. function existUsername($dbusername)
  16. {
  17. global $dbquery,$dbresult,$dbtable;
  18. $dbtable = "users";
  19. $this->_dbusername = $dbusername;
  20. $this->_dbconnect = config::$_dbconnect;
  21. $dbquery = $this->_dbconnect->prepare("SELECT * FROM ".$dbtable." WHERE username = :username");
  22. $dbquery->bindParam(":username",$this->_dbusername);
  23. $dbquery->execute();
  24. $dbresult = $dbquery->rowCount();
  25. if($dbresult == 1)
  26. {
  27. return true;
  28. }
  29. else
  30. {
  31. return false;
  32. }
  33. }
  34. }
  35. ?>

juser.js : 

  2. // JavaScript Document
  3. $(document).ready(function(e) {
  4. $('#username').change(function(){
  5. var existUsername = $('#username').val();
  6. jQuery.ajax({
  7. type: "POST",
  8. url: "index.php",
  9. data: existUsername,
  10. cache: false,
  11. success: function(response)
  12. {
  13. if(response)
  14. {
  15. alert("Ok . ");
  16. }
  17. if(!response)
  18. {
  19. alert("No . ");
  20. }
  21. }
  22. });
  23. });
  24. });
when i enter the username : saeed Ø� (username is not in the database) -> alert ok . 
when i enter the username : admin (username in the datdabase) ->alert ok .
 what is the problem ?