Constructs a new $form from the information in $form_state.
This is the key function for making multi-step forms advance from step to step. It is called by drupal_process_form() when all user input processing, including calling validation and submission handlers, for the request is finished. If a validate or submit handler set $form_state['rebuild'] to TRUE, and if other conditions don't preempt a rebuild from happening, then this function is called to generate a new $form, the next step in the form workflow, to be returned for rendering.
Ajax form submissions are almost always multi-step workflows, so that is one common use-case during which form rebuilding occurs. See ajax_form_callback() for more information about creating Ajax-enabled forms.
$form_id: The unique string identifying the desired form. If a function with that name exists, it is called to build the form array. Modules that need to generate the same form (or very similar forms) using different $form_ids can implement hook_forms(), which maps different $form_id values to the proper form constructor function. Examples may be found in node_forms() and search_forms().
$form_state: A keyed array containing the current state of the form.
$old_form: (optional) A previously built $form. Used to retain the #build_id and #action properties in Ajax callbacks and similar partial form rebuilds. The only properties copied from $old_form are the ones which both exist in $old_form and for which $form_state['rebuild_info']['copy'][PROPERTY] is TRUE. If $old_form is not passed, the entire $form is rebuilt freshly. 'rebuild_info' needs to be a separate top-level property next to 'build_info', since the contained data must not be cached.
The newly built form.
function drupal_rebuild_form($form_id, &$form_state, $old_form = NULL) {
$form = drupal_retrieve_form($form_id, $form_state);
// If only parts of the form will be returned to the browser (e.g., Ajax or
// RIA clients), or if the form already had a new build ID regenerated when it
// was retrieved from the form cache, reuse the existing #build_id.
// Otherwise, a new #build_id is generated, to not clobber the previous
// build's data in the form cache; also allowing the user to go back to an
// earlier build, make changes, and re-submit.
// @see drupal_prepare_form()
$enforce_old_build_id = isset($old_form['#build_id']) && !empty($form_state['rebuild_info']['copy']['#build_id']);
$old_form_is_mutable_copy = isset($old_form['#build_id_old']);
if ($enforce_old_build_id || $old_form_is_mutable_copy) {
$form['#build_id'] = $old_form['#build_id'];
if ($old_form_is_mutable_copy) {
$form['#build_id_old'] = $old_form['#build_id_old'];
}
}
else {
if (isset($old_form['#build_id'])) {
$form['#build_id_old'] = $old_form['#build_id'];
}
$form['#build_id'] = 'form-' . drupal_random_key();
}
// #action defaults to request_uri(), but in case of Ajax and other partial
// rebuilds, the form is submitted to an alternate URL, and the original
// #action needs to be retained.
if (isset($old_form['#action']) && !empty($form_state['rebuild_info']['copy']['#action'])) {
$form['#action'] = $old_form['#action'];
}
drupal_prepare_form($form_id, $form, $form_state);
// Caching is normally done in drupal_process_form(), but what needs to be
// cached is the $form structure before it passes through form_builder(),
// so we need to do it here.
// @todo For Drupal 8, find a way to avoid this code duplication.
if (empty($form_state['no_cache'])) {
form_set_cache($form['#build_id'], $form, $form_state);
}
// Clear out all group associations as these might be different when
// re-rendering the form.
$form_state['groups'] = array();
// Return a fully built form that is ready for rendering.
return form_builder($form_id, $form, $form_state);
}