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function is not described

WP_Network::get_main_site_id() WP 4.9.0

Returns the main site ID for the network.

Internal method used by the magic getter for the 'blog_id' and 'site_id' properties.

This is a method of the class: WP_Network

Hooks from the method
Return

Int. The ID of the main site.

Usage

$WP_Network = new WP_Network();
$var = $WP_Network->get_main_site_id();

Changelog

Since 4.9.0 Introduced.

Code of WP Network::get main site id: wp-includes/class-wp-network.php WP 5.4.1

<?php
private function get_main_site_id() {
	/**
	 * Filters the main site ID.
	 *
	 * Returning a positive integer will effectively short-circuit the function.
	 *
	 * @since 4.9.0
	 *
	 * @param int|null   $main_site_id If a positive integer is returned, it is interpreted as the main site ID.
	 * @param WP_Network $network      The network object for which the main site was detected.
	 */
	$main_site_id = (int) apply_filters( 'pre_get_main_site_id', null, $this );
	if ( 0 < $main_site_id ) {
		return $main_site_id;
	}

	if ( 0 < (int) $this->blog_id ) {
		return (int) $this->blog_id;
	}

	if ( ( defined( 'DOMAIN_CURRENT_SITE' ) && defined( 'PATH_CURRENT_SITE' ) && DOMAIN_CURRENT_SITE === $this->domain && PATH_CURRENT_SITE === $this->path )
		|| ( defined( 'SITE_ID_CURRENT_SITE' ) && SITE_ID_CURRENT_SITE == $this->id ) ) {
		if ( defined( 'BLOG_ID_CURRENT_SITE' ) ) {
			$this->blog_id = (string) BLOG_ID_CURRENT_SITE;

			return (int) $this->blog_id;
		}

		if ( defined( 'BLOGID_CURRENT_SITE' ) ) { // Deprecated.
			$this->blog_id = (string) BLOGID_CURRENT_SITE;

			return (int) $this->blog_id;
		}
	}

	$site = get_site();
	if ( $site->domain === $this->domain && $site->path === $this->path ) {
		$main_site_id = (int) $site->id;
	} else {
		$cache_key = 'network:' . $this->id . ':main_site';

		$main_site_id = wp_cache_get( $cache_key, 'site-options' );
		if ( false === $main_site_id ) {
			$_sites       = get_sites(
				array(
					'fields'     => 'ids',
					'number'     => 1,
					'domain'     => $this->domain,
					'path'       => $this->path,
					'network_id' => $this->id,
				)
			);
			$main_site_id = ! empty( $_sites ) ? array_shift( $_sites ) : 0;

			wp_cache_add( $cache_key, $main_site_id, 'site-options' );
		}
	}

	$this->blog_id = (string) $main_site_id;

	return (int) $this->blog_id;
}