Drupal: Using #ajax on a form

<?php/** * Implementation of hook_form_alter *  * @param unknown $form * @param unknown $form_state * @param unknown $form_id* @author DrupalD http://twtitter.com/DrupalD */function mymodule_form_alter(&$form, &$form_state, $form_id) {  switch($form_id) {      case 'incident_form_node_form':        $form['client_er'] = array(            '#type' => 'item',            '#title' => t('Client\'s Email Recipients'),            '#description' => t('Once you select a client, you will see the email recipient for that client listed here'),            '#weight' => -49,        );                $form['field_client']['und']['#ajax'] = array(          'callback' => '_mymodule_recipitn_js',          'wrapper' => 'edit-client-er',        );  }}/** * _mymodule_recipitn_js *  * Return email recipint * @author DrupalD http://twtitter.com/DrupalD */function _mymodule_recipitn_js($form, $form_state){  $am__recipient = array();    //Get the selected client  $sn__client_nid = $form_state['triggering_element']['#value'];    //Find all the email recipints for that client  $am__recipient = _get_node_email_recipient($sn__client_nid);  if (!empty($am__recipient)) {     $form['client_er'] = array(        '#type' => 'item',        '#markup' => implode('<br />', $am__recipient),        '#title' => t('Client\'s Email Recipients'),        '#description' => t('Here are all the email recipients from selected client.             If you do not add any new email recipints in #email_recipients field below, above details will be used for this incident', array('#email_recipients' => t('Email Recipients'))),        '#prefix' => '<div id="edit-client-er">',          '#suffix' => '<div id="edit-client-er">',      );    return  array(       '#type' => 'ajax',       '#commands' => array(         ajax_command_replace("#edit-client-er", render($form['client_er'])),       ),      );   }  else {    return t('No email recipients found for this client');  }}/** * _get_node_email_recipient *  * @param unknown $sn__client_nid * @author DrupalD http://twtitter.com/DrupalD */function _get_node_email_recipient($sn__client_nid) {  $result = db_select('field_data_field_email_recipients', 'fer')            ->fields('fer', array('field_email_recipients_email'))            ->condition('entity_id', $sn__client_nid, '=')            ->execute();  while ($am__email_recipient = $result->fetchAssoc()) {    $am__recipient[] = $am__email_recipient['field_email_recipients_email'];  }  return $am__recipient;}?>