Drupal - Generating <button type="submit"> with the form API

As an addition, just in case someone runs into the same trouble as I did - when using the #markup or #prefix/#suffix trick on a form's actions group, the submit callback function won't be called at all, unless a submit type element is present. My workaround was like this:

$form['actions']['submit'] = array
(
    '#type' => 'submit',
    '#value' => '',
    '#attributes' => array( 'style' => array( 'display: none' )), // hide the input field
    '#submit' => array( 'my_callback_for_the_form_submit' ),
    '#prefix' => '<button type="submit" class="btn btn-primary">Add <i class="fa fa-plus-square-o">',
    '#suffix' => '</i></button>',
);

That way you can use custom HTML for submit action groups.

In D7 would recommend:

$form['custom-form'] = array(
  '#prefix' => '<button type="submit">',
  '#suffix' => '</button>',
  '#markup' => '<span>' . t('Login') . '</span>',
);

That way you can replace the #markup in an alter function later if needed, without having to rebuild the button HTML.

To add some custom tag you can use the following snippets:

// Drupal 6.
$form = array();

// Other elements.

$form['custom-form'] = array(
    '#value' => '<button type="submit" class="clearfix"><span>Login</span></button>',
);

// Drupal 7.
$form = array();

// Other elements.

$form['custom-form'] = array(
    '#markup' => '<button type="submit" class="clearfix"><span>Login</span></button>',
);