Determining a Widget Instance and Sidebar Location?

The widget_display_callback hook takes 3 arguments as we can see in /wp-includes/widgets.php: ^{(line 180, WP 3.4.2)}

• $instance = apply_filters('widget_display_callback', $instance, $this, $args);

So, to make a complete hook, the code is:

// Priority = 10 // Arguments = 3
add_filter('widget_display_callback', 'wpse_57546_widget_display_callback', 10, 3);

// Using "$this" as an argument produces errors, 
// as it is a reference to the current object
// see http://stackoverflow.com/q/1523479/1287812
// hence it is renamed to "$thiz"
function wpse_57546_widget_display_callback( $instance, $thiz, $args ) 
{
    /* Some Code */
    return $instance;
}

Using FirePHP, we see that this filter runs in all widgets being displayed in the frontend, and the following are the values of the widget “Categories”:

$instance = array(
    ['title'] =>
    ['count'] => 0
    ['hierarchical'] => 0
    ['dropdown'] => 0
)

$thiz = WP_Widget_Categories(
    id_base="categories"
    name="Categories"
    widget_options = array(
        ['classname'] => 'widget_categories'
        ['description'] => 'A list or dropdown of categories'
    )
    control_options = array(
        ['id_base'] => 'categories'
    )
    number = 2
    id = 'categories-2'
    updated =
    option_name="widget_categories"
)

$args = array(
    ['name'] => 'Main Sidebar'
    ['id'] => 'sidebar-1'
    ['description'] =>
    ['class'] =>
    ['before_widget'] => '<aside id="categories-2" class="widget widget_categories">'
    ['after_widget'] => '</aside>'
    ['before_title'] => '<h3 class="widget-title">'
    ['after_title'] => '</h3>'
    ['widget_id'] => 'categories-2'
    ['widget_name'] => 'Categories'
)

The values you are looking for are inside $args:

$args['id']
$args['widget_id']

And to access $thiz values, you’d use:

$thiz->id_base
$thiz->widget_options['classname']