_register_widget_form_callback( int|string $id, string $name, callable $form_callback, array $options = array() )

function _register_widget_form_callback($id, $name, $form_callback, $options = array()) {
	global $wp_registered_widget_controls;

	$id = strtolower($id);

	if ( empty($form_callback) ) {
		unset($wp_registered_widget_controls[$id]);
		return;
	}

	if ( isset($wp_registered_widget_controls[$id]) && !did_action( 'widgets_init' ) )
		return;

	$defaults = array('width' => 250, 'height' => 200 );
	$options = wp_parse_args($options, $defaults);
	$options['width'] = (int) $options['width'];
	$options['height'] = (int) $options['height'];

	$widget = array(
		'name' => $name,
		'id' => $id,
		'callback' => $form_callback,
		'params' => array_slice(func_get_args(), 4)
	);
	$widget = array_merge($widget, $options);

	$wp_registered_widget_controls[$id] = $widget;
}