is_main_site() │ WP 3.0.0

Determines whether the current/specified site is the main site of the network (MU build).

Used in the WordPress multisite installation (MU). In a single-site installation it always returns true.

Works at an early stage of WordPress loading, even before the constant SHORTINIT.

Uses the global variable $current_site

Uses: get_current_blog_id(), is_multisite()
1 time — 0.000013 sec (very fast) | 50000 times — 0.045192 sec (speed of light)

No Hooks.

Returns

true|false. Will return true if $site_id is the main site of the network or MU is not used. In other cases will return false.

Usage

is_main_site( $site_id );
$site_id(int)
Site ID to check.
Default: null (current site)
$network_id(int)
Network ID whose main site is being checked.
Default: null (current network)

Examples

0

#1 Do anything only if this is the main site of the network 

if ( is_main_site() ) {
	// this is the main network site
}

Changelog

Since 3.0.0 Introduced.
Since 4.9.0 The $network_id parameter was added.

is_main_site() code WP 6.9

wp-includes/functions.php 
function is_main_site( $site_id = null, $network_id = null ) {
	if ( ! is_multisite() ) {
		return true;
	}

	if ( ! $site_id ) {
		$site_id = get_current_blog_id();
	}

	$site_id = (int) $site_id;

	return get_main_site_id( $network_id ) === $site_id;
}

Multisite