1 system.api.php hook_forms($form_id, $args)

Map form_ids to form builder functions.

By default, when backdrop_get_form() is called, the system will look for a function with the same name as the form ID, and use that function to build the form. If no such function is found, Backdrop calls this hook. Modules implementing this hook can then provide their own instructions for mapping form IDs to constructor functions. As a result, you can map multiple form IDs to a single form constructor (referred to as a 'base' form).

Using a base form can help to avoid code duplication, by allowing many similar forms to use the same code base. Another benefit is that it becomes much easier for other modules to apply a general change to the group of forms; hook_form_BASE_FORM_ID_alter() can be used to alter multiple forms at once by directly targeting the shared base form.

Two example use cases where base forms may be useful are given below.

First, you can use this hook to tell the form system to use a different function to build certain forms in your module; this is often used to define a form "factory" function that is used to build several similar forms. In this case, your hook implementation will likely ignore all of the input arguments. See node_forms() for an example of this. Note, node_forms() is the hook_forms() implementation; the base form itself is defined in node_form().

Second, you could use this hook to define how to build a form with a dynamically-generated form ID. In this case, you would need to verify that the $form_id input matched your module's format for dynamically-generated form IDs, and if so, act appropriately.

Third, forms defined in classes can be defined this way.


$form_id: The unique string identifying the desired form.

$args: An array containing the original arguments provided to backdrop_get_form() or backdrop_form_submit(). These are always passed to the form builder and do not have to be specified manually in 'callback arguments'.

Return value

An associative array whose keys define form_ids and whose values are an: associative array defining the following keys:

  • callback: The callable returning the form array. If it is the name of the form builder function then this will be used for the base form ID, for example, to target a base form using hook_form_BASE_FORM_ID_alter(). Otherwise use the base_form_id key to define the base form id.
  • callback arguments: (optional) Additional arguments to pass to the function defined in 'callback', which are prepended to $args.
  • base_form_id: The base form id can be specified explicitly. This is required when callback is not the name of a function.
  • wrapper_callback: (optional) Any callable to invoke before the form builder defined in 'callback' is invoked. This wrapper callback may prepopulate the $form array with form elements, which will then be already contained in the $form that is passed on to the form builder defined in 'callback'. For example, a wrapper callback could setup wizard-alike form buttons that are the same for a variety of forms that belong to the wizard, which all share the same wrapper callback.

Related topics


core/modules/system/system.api.php, line 1411
Hooks provided by Backdrop core and the System module.


function hook_forms($form_id, $args) {
  // Reroute the (non-existing) $form_id 'my_module_first_form' to
  // 'my_module_main_form'.
  $forms['my_module_first_form'] = array(
    'callback' => 'my_module_main_form',

  // Reroute the $form_id and prepend an additional argument that gets passed to
  // the 'my_module_main_form' form builder function.
  $forms['my_module_second_form'] = array(
    'callback' => 'my_module_main_form',
    'callback arguments' => array('some parameter'),

  // Reroute the $form_id, but invoke the form builder function
  // 'my_module_main_form_wrapper' first, so we can prepopulate the $form array
  // that is passed to the actual form builder 'my_module_main_form'.
  $forms['my_module_wrapped_form'] = array(
    'callback' => 'my_module_main_form',
    'wrapper_callback' => 'my_module_main_form_wrapper',

  // Build a form with a static class callback.
  $forms['my_module_class_generated_form'] = array(
    // This will call: MyClass::generateMainForm().
    'callback' => array('MyClass', 'generateMainForm'),
    // The base_form_id is required when the callback is a static function in
    // a class. This can also be used to keep newer code backwards compatible.
    'base_form_id' => 'my_module_main_form',

  return $forms;