return system_settings_from($form);

<?php
/**
 * Form manipulation through AJAX.
 */
function ajax_example_autocheckboxes($form, &$form_state) {

  // Since the form builder is called after every AJAX request, we rebuild
  // the form based on $form_state.
  $num_checkboxes = !empty($form_state['values']['howmany_select']) ? $form_state['values']['howmany_select'] : 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' => $num_checkboxes,
    '#ajax' => array(
      'callback' => 'ajax_example_autocheckboxes_callback',
      'wrapper' => 'checkboxes-div',
      'effect' => 'slide',
    ),
  );

  ...

  for ($i = 1; $i <= $num_checkboxes; $i++) {
    $form['checkboxes_fieldset']["checkbox$i"] = array(
      '#type' => 'checkbox',
      '#title' => "Checkbox $i",
    );
  }

  ...

  return $form;
}


/**
 * Callback for autocheckboxes.
 *
 * @return array
 *   Renderable array (the checkboxes fieldset)
 */
function ajax_example_autocheckboxes_callback($form, $form_state) {
  return $form['checkboxes_fieldset'];
}