get_main_site_id() WordPress Function

function get_main_site_id( $network_id = null ) {
	if ( ! is_multisite() ) {
		return get_current_blog_id();
	}

	$network = get_network( $network_id );
	if ( ! $network ) {
		return 0;
	}

	return $network->site_id;
}
Uses	Description
wp-includes/ms-network.php:get_network()	Retrieves network data given a network ID or network object.
wp-includes/load.php:is_multisite()	If Multisite is enabled.
wp-includes/load.php:get_current_blog_id()	Retrieve the current site ID.
Used By	Description
wp-includes/ms-load.php:ms_load_current_site_and_network()	Identifies the network and site of a requested domain and path and populates the corresponding network and site global objects as part of the multisite bootstrap process.
wp-includes/functions.php:is_main_site()	Determine whether a site is the main site of the current network.