<?php
/<strong>
 * AJAX-enabled select element causes replacement of a set of checkboxes
 * based on the selection.
 */
function ajax_example_autocheckboxes($form, &$form_state) {

  $default = !empty($form_state['values']['howmany']) ? $form_state['values']['howmany'] : 1;

  $form['howmany_select'] = array(
    '#title' => t('How many checkboxes do you want?'),
    '#type' => 'select',
    '#options' => array(1 => 1, 2 => 2, 3 => 3, 4 => 4),
    '#default_value' => $default,
    '#ajax' => array(
      'callback' => 'ajax_example_autocheckboxes_callback',
      'wrapper' => 'checkboxes-div',
      'method' => 'replace',
      'effect' => 'fade',
    ),

  );


  $form['checkboxes_fieldset'] = array(
    '#title' => t("Generated Checkboxes"),
    // The prefix/suffix provide the div that we're replacing, named by
    // #ajax['wrapper'] above.
    '#prefix' => '<div id="checkboxes-div">',
    '#suffix' => '</div>',
    '#type' => 'fieldset',
    '#description' => t('This is where we get automatically generated checkboxes'),
  );

  $num_checkboxes = !empty($form_state['values']['howmany_select']) ? $form_state['values']['howmany_select'] : 1;
  for ($i=1; $i<=$num_checkboxes; $i++) {
    $form['checkboxes_fieldset']["checkbox$i"] = array(
      '#type' => 'checkbox',
      '#title' => "Checkbox $i",
    );
  }

  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Submit'),
  );

  return $form;
}

/</strong>
 * Callback element needs only select the portion of the form to be updated.
 * Since #ajax['callback'] return can be HTML or a renderable array (or an
 * array of commands), we can just return a piece of the form.
 */
function ajax_example_autocheckboxes_callback($form, $form_state) {
  return $form['checkboxes_fieldset'];
}
?>