I am building a form that can either begin on the home page or at the '/apply' url. I used this tutorial to build the multi-step form. The form works great if I start from '/apply' and begin at the very beginning of the form process. The form does not work if I start from the home page and get redirected to the second step of the form, then try to submit that second step. This is the error message I receive when I try to submit that second page of the form when I started on the home page:

PHP Fatal error:  Cannot create references to/from string offsets nor overloaded objects in /Applications/MAMP/htdocs/drupal/includes/common.inc on line 6548
PHP Stack trace:
PHP   1. {main}() /Applications/MAMP/htdocs/drupal/index.php:0
PHP   2. menu_execute_active_handler() /Applications/MAMP/htdocs/drupal/index.php:21
PHP   3. call_user_func_array() /Applications/MAMP/htdocs/drupal/includes/menu.inc:517
PHP   4. drupal_get_form() /Applications/MAMP/htdocs/drupal/includes/menu.inc:517
PHP   5. drupal_build_form() /Applications/MAMP/htdocs/drupal/includes/form.inc:131
PHP   6. drupal_process_form() /Applications/MAMP/htdocs/drupal/includes/form.inc:374
PHP   7. form_builder() /Applications/MAMP/htdocs/drupal/includes/form.inc:842
PHP   8. form_builder() /Applications/MAMP/htdocs/drupal/includes/form.inc:1857
PHP   9. form_builder() /Applications/MAMP/htdocs/drupal/includes/form.inc:1857
PHP  10. form_builder() /Applications/MAMP/htdocs/drupal/includes/form.inc:1857
PHP  11. _form_builder_handle_input_element() /Applications/MAMP/htdocs/drupal/includes/form.inc:1795
PHP  12. drupal_array_set_nested_value() /Applications/MAMP/htdocs/drupal/includes/form.inc:2003

I am attempting to continue the multi-step form from the home page to the '/apply' page by saving the $form_state['storage'] array in the session, then reading it out of the session in the custom_module_primary_form function.

//Save form 'storage' in $_SESSION
function custom_module_first_form_submit($form, &$form_state) {
  if(request_path() == '') {
    $form_state['redirect'] = array('apply');
    $_SESSION['storage'] = $form_state['storage'];
    $form_state['rebuild'] = FALSE;

This is the modified custom_module_primary_form function from the tutorial.

//Read form 'storage' out of $_SESSION
function custom_module_primary_form($form, &$form_state) {
  // Check to see if anything has been stored.
  if ($form_state['rebuild']) {
    $form_state['input'] = array();

  if (empty($form_state['storage'])) {
    if(isset($_SESSION['storage'])) {
      $form_state['submitted'] = TRUE;
      $form_state['rebuild'] = TRUE;
      $form_state['storage'] = $_SESSION['storage'];

    } else {
      // No step has been set so start with the first.
      $form_state['storage'] = array(
        'step' => 'custom_module_first_form',

  // Return the current form
  $function = $form_state['storage']['step'];
  $form = $function($form, $form_state);
  return $form;

I would recommend reviewing the Drupal Contrib examples project:

This project aims to provide high-quality, well-documented API examples for a broad range of Drupal core functionality.

It has an example in the form's submodule that provides you some working stub / skeleton code that you can learn, modify or apply to your own project.

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