PHP da hatalar var, mesela :

exit
degil, boyle olmali
exit();


if (!$_POST['password'] == $_POST['repassword'])

degil, boyle olmali

if ($_POST['password'] != $_POST['repassword'])

$kayit = $insert->execute(array(
'username' = $_POST['username'],
'email' = $_POST['email'],
'password' = $_POST['password'],
'repassword' = $_POST['repassword']
));

degil, boyle olmali

$kayit = $insert->execute(array(
'username' => $_POST['username'],
'email' => $_POST['email'],
'password' => $_POST['password'],
'repassword' => $_POST['repassword']
));