webform_alter_ajax.php

<?php
// playground.module

function playground_form_alter(&$form, &$form_state, $form_id) {
  switch($form_id) {
    case 'webform_client_form_1':
      global $user;
      if ($user->uid < 0) {
        $form['submitted']['username']['#access'] = FALSE;
      }
      $form['submitted']['username']['#ajax'] = array(
        'wrapper' => 'webform-client-form',
        'method' => 'replace',
        'effect' => 'fade',
        'event' => 'blur',
        'callback' => '_playground_check_username',
      );
      break;
  }
}

function _playground_check_username($form, &$form_state) {
  $parents = $form_state['triggering_element']['#array_parents'];
  array_pop($parents);
  $element = &drupal_array_get_nested_value($form, $parents);

  $exists = FALSE;
  // $username = $element['#value']['username'];
  $username= $form_state['values']['submitted']['username'];

  if (!empty($username)) {
    $exists = db_query('SELECT name FROM {users} WHERE name = :entered', array(':entered' => $username))->fetchField();
  }

  $commands = array();
  if ($exists) {
  form_error($element, 'Kullanıcı adı kayıtlı, lütfen başka bir kullanıcı adı seçin');
  $error_selector = '.webform-client-form';
  $commands[] = ajax_command_replace($error_selector, theme('status_messages'). drupal_render($element) );
  // Use below commands if you want to display in message region.
  // $commands[] = ajax_command_replace($error_selector, drupal_render($element) );
  // $commands[] = ajax_command_remove('#messages');
  // $commands[] = ajax_command_before('#main-wrapper', '<div id="messages"><div class="section clearfix">' . theme('status_messages') . '<div></div>');

  return array('#type' => 'ajax', '#commands' => $commands);
  }
}

  $commands[] = ajax_command_replace($error_selector, drupal_render($element) );
  $commands[] = ajax_command_remove('#messages');
  $commands[] = ajax_command_before('#main-wrapper', '<div id="messages"><div class="section clearfix">' . theme('status_messages') . '<div></div>');