Get Root Domain of a WordPress Multisite Network

The functions below can be used to get the root domain of a WordPress site. If the site is a multisite network and you’re on http://www.site1.example.com/page-1/, calling km_get_root_domain() would return example.com for instance. It also works on single-site WP installations.

/**
 * Get the root domain of the site/network.
 *
 * @return string|bool The root domain or false on failure.
 */
function km_get_root_domain() {

	$url_parts = parse_url( km_get_main_site_url() );

	if ( $url_parts && isset( $url_parts['host'] ) ) {
		return $url_parts['host'];
	}

	return false;
}

/*
 * Get the home URL of the main site.
 *
 * @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();
}