How do you implement a custom submit handler in hook_form_alter()?

To replace/override the existing handler in Drupal 9 this worked for me:

**
 * Implements hook_form_FORM_ID_alter()
 */
function MODULE_NAME_form_FORM_ID_alter(&$form, $form_state, $form_id) {

  $form['submit']['#submit'][] = 'custom_submit';
}

function custom_submit($form, $form_state) {
  Drupal::messenger()->addStatus(t('Submit overriden by custom_submit!'));
}

Here's how to attach a custom submit handler to the article node add/edit form in Drupal 8:

<?php

use DrupalCoreFormFormStateInterface;

/**
 * Implements hook_form_alter().
 */
function my_module_form_alter(&$form, FormStateInterface $form_state, $form_id) {

  // Use this to reveal the form id.
  //drupal_set_message($form_id);

  // Use this with the devel module to inspect the button action(s).
  //kint($form['actions']);

  switch ($form_id) {

    case 'node_article_form':      // New article nodes.
    case 'node_article_edit_form': // Existing article nodes.

      // Attach our custom submit handler.
      $form['actions']['publish']['#submit'][] = 'my_module_node_article_form_submit';
      break;

  }

}

function my_module_node_article_form_submit($form, FormStateInterface $form_state) {
  drupal_set_message('Running custom submit handler...');
}

I was not able to successfully append a custom submit handler to $form['#submit'] and have the function fire properly. I had to explicitly attach it to the Save and publish button when creating a new article, and attach it to the Save and keep published button when editing an existing article.

Besides publish, here are some of the other button actions available:

unpublish
preview
delete

You can add how many submit handler you want using the following code:

 $form['actions']['submit']['#submit'][] = 'mymodule_what_ever_function';

If you want to add a submit handler after the default submit handler, (a submit handler that will be called after the submit callback), you can use the following.

$form['#submit'][1] = test_function;

To remove the submit handler:

unset($form["actions"]['submit']);

The behavior is either changed or differs from case to case. With the Node Delete Multiple confirmation form

$form['actions']['submit']['#submit'][] = 'mymodule_upload_enabled_types_submit';

@Clive's code causes Drupal to ignore the main handler. In this case add the handler to the $form['#submit'] section:

$form['#submit'][] = 'mymodule_upload_enabled_types_submit';

According to the example in the documentation it's the same as for Drupal 7:

$form['actions']['submit']['#submit'][] = 'mymodule_upload_enabled_types_submit';

You can find similar (working) logic in

• contact_form_user_form_alter()
• editor_form_filter_format_form_alter()
• locale_form_language_admin_edit_form_alter()

And a few others besides.