switch_to_blog and restore_current_blog to make it easier, while using the standard WordPress APIs.restore_current_blog undos the last call to switch_to_blog, but only by one step, the calls are not nestable, so always call restore_current_blog before calling switch_to_blog again.get_sites( $args) function, available since version 4.6 of WordPress. The get_sites( $args) function accepts an array or query string of site query parameters specifying the kind of sites you are looking for. For versions 3.7 to 4.6 a similar function wp_get_sites( $args ) was available.COOKIE_DOMAIN constant to an empty string in your wp-config.php:$current_site->domain, which could cause issues in some situations.