Drupal7: Update form element via AJAX.

example.module

<?php

function example_form($form, &$form_state) {
  $form['foo'] = array(
    '#type' => 'select',
    '#title' => t('Foo'),
    '#options' => array('one', 'two', 'three'),
    '#ajax' => array(
      'callback' => 'example_foo_callback',
      'wrapper' => 'bar-sections'
    )
  );
  
  if ($form_state['values']['foo']) {
    $form['bar'] = array(
      '#id' => 'bar-sections',
      '#markup' => $form_state['values']['foo'],
    );
  }
}

function example_foo_callback($form, $form_state) {
  return $form['bar'];
}