Cacher is the code snippet organizer for pro developers

//ref  : https://www.deckfifty.com/blog/2012-09/drupal-form-api-using-ajax

/**
 * Implements hook_menu().
 */
function deckfifty_menu() {
  $items['example'] = array(
    'title' => 'Example Page',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('deckfifty_example_form'),
    'access arguments' => array('access content'),
  );
 
  return $items;
}




function deckfifty_example_form($form, &$form_state) {
  $form['fox_color'] = array(
    '#title' => t('Specify the color of the fox:'),
    '#type' => 'select',
    '#options' => drupal_map_assoc(array('', 'brown', 'black', 'white')),
    '#ajax' => array(
      'callback' => 'deckfifty_fox_ajax_callback',
      'wrapper' => 'deckfifty_fox_ajax_wrapper',
    ),
  );
  $form['fox_wrapper'] = array(
    '#type' => 'markup',
    '#prefix' => '<div id="deckfifty_fox_ajax_wrapper">',
    '#suffix' => '</div>',
  );
 
  if( (isset($form_state['values']['fox_color'])) && ($form_state['values']['fox_color'] != '') ) {
    $form['fox_wrapper']['#markup'] = t('The quick @color fox jumps over the lazy dog.', array('@color' => $form_state['values']['fox_color']));
  }
 
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Save'),
  );
 
  return $form;
}



function deckfifty_fox_ajax_callback($form, $form_state) {
 return $form['fox_wrapper'];
}