Source

File: wp-includes/widgets.php

function _register_widget_form_callback( $id, $name, $form_callback, $options = array(), ...$params ) {
	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'   => $params,
	);
	$widget = array_merge( $widget, $options );

	$wp_registered_widget_controls[ $id ] = $widget;
}