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action is not described

shutdown action-hook . WP 1.2.0

Fires just before PHP shuts down execution.

Usage

add_action( 'shutdown', 'action_function_name_218' );
function action_function_name_218(){
	// action...
}

Where the hook is called

shutdown_action_hook()
wp-includes/load.php 684
do_action( 'shutdown' );

Where the hook is used (in WP core)

wp-includes/default-filters.php 337
add_action( 'shutdown', 'wp_ob_end_flush_all', 1 );