get_id_from_blogname( string $slug )

function get_id_from_blogname( $slug ) {
	$current_network = get_network();
	$slug = trim( $slug, '/' );

	if ( is_subdomain_install() ) {
		$domain = $slug . '.' . preg_replace( '|^www\.|', '', $current_network->domain );
		$path = $current_network->path;
	} else {
		$domain = $current_network->domain;
		$path = $current_network->path . $slug . '/';
	}

	$site_ids = get_sites( array(
		'number' => 1,
		'fields' => 'ids',
		'domain' => $domain,
		'path' => $path,
	) );

	if ( empty( $site_ids ) ) {
		return null;
	}

	return array_shift( $site_ids );
}