In the PHPdoc of the function
WP_Widget::update() located under
wp-includes/widgets.php it says the following:
/** * ... * This function should check that $new_instance is set correctly. * The newly calculated value of $instance should be returned. * If "false" is returned, the instance won't be saved/updated. * ... */
It is unclear to me why the
$old_instance is needed when updating the values and how can it be used to validate that the
$new_instance is set correctly.