Wim Leers codesnippet for Drupal AHAH

<?php
      // Build our new form element.
      $form_element = _mymodule_add_something_to_form();

      // Build the new form.
      $form_state = array('submitted' => FALSE);

      $form_build_id = $_POST['form_build_id'];

      // Add the new element to the stored form. Without adding the element
      // to the form, Drupal is not aware of this new elements existence and
      // will not process it. We retreive the cached form, add the element,
      // and resave.

      $form = form_get_cache($form_build_id, $form_state);
 
      $form['somewhere']['very']['deep'] = $form_element;

      form_set_cache($form_build_id, $form, $form_state);

      $form += array(

      '#post' => $_POST,

      '#programmed' => FALSE,

      );

      // Rebuild the form.
      $form = form_builder('mymodule_someform', $form, $form_state);
 
      // Render the new output.
      $subform = $form['somewhere']['choice'];
      $output = theme('status_messages') . drupal_render($subform);
      drupal_json(array('status' => TRUE, 'data' => $output));
?>

Source: http://wimleers.com/blog/ahah-helper-module

This is how you can set and send a drupal_set_message() via Drupal AHAH. This code is placed first in the function to return the result in the AHAH call.

<?php
    if ($start_time > $stop_time) {
      drupal_set_message(t("Start date has to be before Stop date"), "error");
      return $output = theme('status_messages');
    }
?>

I have used it like this

<?php
function cdt_stats_navigate($direction){

      // Build our new form element.
      $form_element = cdt_stats_calendarmonth_form($form_state, $direction, $_POST['timestamp'], $_POST['project_id'], $_POST['view']);

      // Build the new form.
      $form_state = array('submitted' => FALSE);

      $form_build_id = $_POST['form_build_id'];

      $form = form_get_cache($form_build_id, $form_state);

      $form['linkpair']['info'] = $form_element;

      form_set_cache($form_build_id, $form, $form_state);

      $form += array(
        '#post' => $_POST,
        '#programmed' => FALSE,
      );

      // Rebuild the form.
      $form = form_builder('cdt_stats_calendar_form', $form, $form_state, $_POST['project_id'], $_POST['view']);
      // Render the new output.
      $subform = $form['linkpair']['info'];
      $output = theme('status_messages') . drupal_render($subform);
      drupal_json(array('status' => TRUE, 'data' => $output));
}
?>
Knowledge keywords: 
Drupal
AHAH