Get Main Site URL in WordPress Multisite Network

get-main-site-url-in-wordpress-multisite-network.php

<?php

/*
 * Get the home URL of the main site in a multisite network.
 *
 * @return string The home URL of the main site.
 */
function km_get_main_site_url() {

	// This is the current network's information; 'site' is old terminology.
	global $current_site;

	if ( is_multisite() && $current_site ) {
		$main_site_blog_id = $current_site->blog_id;
		return get_home_url( $main_site_blog_id );
	}

	return home_url();
}