Dynamic loading of select list using AJAX

<?php

/**
 * @file
 * dynamic_select_list.module.
 */

/**
 * Implements hook_menu().
 */
function blogs_dynamic_list_menu() {
  $items = array();

  // Put your menu items here.
  $items['blogs/dynamic-list'] = array(
    'title' => 'Dynamic lists',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('blogs_dynamic_list_render_select_list'),
    'access callback' => TRUE,
    'type' => MENU_NORMAL_ITEM,
  );

  return $items;
}

function blogs_dynamic_list_render_select_list($form, &$form_state) {
  $animal_category = array(
    'none' => t('-- None --'),
    'mammal' => t('Mammal'),
    'reptile' => t('Reptile'),
    'aves' => t('Aves'),
    'amphibia' => t('Amphibia'),
  );

  $mammal = array(
    'tiger' => t('Tiger'),
    'cat' => t('Cat'),
    'zebra' => t('Zebra'),
    'panda' => t('Panda'),
    'squirrel' => t('Squirrel'),
  );

  $reptile = array(
    'snake' => t('Snake'),
    'crocodile' => t('Crocodile'),
    'turtle' => t('Turtle'),
    'lizard' => t('Lizard'),
  );

  $aves = array(
    'parrot' => t('Parrot'),
    'eagle' => t('Eagle'),
    'owl' => t('Owl'),
    'peacock' => t('Peacock'),
  );

  $amphibia = array(
    'frog' => t('Frog'),
    'salamander' => t('Salamander'),
  );

  // Check the value of parent select list and populate the value of child
  // select list according to that.
  $default = !empty($form_state['values']['parent_select_list']) ? $form_state['values']['parent_select_list'] : 'none';

  switch ($default) {
    case 'mammal':
      $child_select_options = $mammal;
      break;

    case 'reptile':
      $child_select_options = $reptile;
      break;

    case 'aves':
      $child_select_options = $aves;
      break;

    case 'amphibia':
      $child_select_options = $amphibia;
      break;

    case 'none':
      $child_select_options = array('none' => '-- None --');
      break;
  }

  // Parent select list.
  $form['parent_select_list'] = array(
    '#type' => 'select',
    '#title' => t('Select animal category'),
    '#options' => $animal_category,
    '#default_value' => $animal_category['none'],
    '#description' => t('Select animal class'),
    '#ajax' => array(
      'callback' => 'blog_dynamic_list_select_list_ajax_callback',
      'wrapper' => 'child-select-list-wrapper',
      'event' => 'change',
    ),
  );

  // Child select list.
  $form['child_select_list'] = array(
    '#prefix' => '<div id="child-select-list-wrapper">',
    '#type' => 'select',
    '#title' => t('Select animal'),
    '#options' => $child_select_options,
    '#description' => t('Select animal'),
    '#suffix' => '</div>',
  );

  return $form;
}

/**
 * AJAX callback.
 */
function blog_dynamic_list_select_list_ajax_callback($form, &$form_state) {
  return $form['child_select_list'];
}

<?php

/**
 * @file
 * blogs_dynamic_list_terms.module.
 */

/**
 * Implements hook_menu().
 */
function blogs_dynamic_list_terms_menu() {
  $items = array();

  // Put your menu items here.
  $items['blogs/dynamic-list-terms'] = array(
    'title' => 'Dynamic lists terms',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('blogs_dynamic_list_terms_render_select_list'),
    'access callback' => TRUE,
    'type' => MENU_NORMAL_ITEM,
  );

  return $items;
}

function blogs_dynamic_list_terms_render_select_list($form, &$form_state) {
  // Load the vocabulary. Instead of 'taxonomy_animals' you have to give the
  // machine name of your vocabulary.
  $vocabulary = taxonomy_vocabulary_machine_name_load('taxonomy_animals');

  // Load all the parent terms of the vocabulary.
  // The second parameter 0 means that we wish to generate the whole tree.
  // The third parameter 1 means that we wish to see only the parent terms.
  // Default is 0.
  $parent_terms = taxonomy_get_tree($vocabulary->vid, 0, 1);

  $animal_category = array('none' => '-- None --');
  // Build the parent select list values.
  foreach ($parent_terms as $parent) {
    $animal_category[$parent->tid] = $parent->name;
  }

  // Check the value of parent select list and populate the value of child
  // select list according to that.
  $default = !empty($form_state['values']['parent_select_list']) ? $form_state['values']['parent_select_list'] : 'none';

  if ($default != 'none') {
    // Load all the child terms of the selected parent item.
    // The second parameter tells to load the tree based on the parent term ID.
    // We are ignoring the third parameter since we know that the vocabulary has
    // only two levels.
    $child_terms = taxonomy_get_tree($vocabulary->vid, $default);
    // Build the child select list values.
    foreach ($child_terms as $child) {
      $child_select_options[$child->tid] = $child->name;
    }
  }
  else {
    // No value selected in parent selct list.
    $child_select_options = array('none' => '-- None --');
  }

  // Parent select list.
  $form['parent_select_list'] = array(
    '#type' => 'select',
    '#title' => t('Select animal category'),
    '#options' => $animal_category,
    '#default_value' => $animal_category['none'],
    '#description' => t('Select animal class'),
    '#ajax' => array(
      'callback' => 'blogs_dynamic_list_terms_select_list_ajax_callback',
      'wrapper' => 'child-select-list-wrapper',
      'event' => 'change',
    ),
  );

  // Child select list.
  $form['child_select_list'] = array(
    '#prefix' => '<div id="child-select-list-wrapper">',
    '#type' => 'select',
    '#title' => t('Select animal'),
    '#options' => $child_select_options,
    '#description' => t('Select animal'),
    '#suffix' => '</div>',
  );

  return $form;
}

/**
 * AJAX callback.
 */
function blogs_dynamic_list_terms_select_list_ajax_callback($form, &$form_state) {
  return $form['child_select_list'];
}