dynamic_sidebar_before action-hook . WP 3.9.0

Fires before widgets are rendered in a dynamic sidebar.

Note: The action also fires for empty sidebars, and on both the front end and back end, including the Inactive Widgets sidebar on the Widgets screen.

Usage

add_action( 'dynamic_sidebar_before', 'action_function_name_3473', 10, 2 );
function action_function_name_3473( $index, $has_widgets ){
	// action...
}

$index(int/string)

Index, name, or ID of the dynamic sidebar.

$has_widgets(true/false)

Whether the sidebar is populated with widgets.
Default: true

Where the hook is called

do_action( 'dynamic_sidebar_before', $index, true );

do_action( 'dynamic_sidebar_before', $index, false );

Where the hook is used (in WP core)

add_action( 'dynamic_sidebar_before', array( $this, 'start_dynamic_sidebar' ) );