WP Maintenance Mode

wp_admin_mode

// Temp Maintenance - with http response 503 (Service Temporarily Unavailable)
// This will only block users who are NOT an administrator from viewing the website.
function wp_maintenance_mode(){
    if(!current_user_can('edit_themes') || !is_user_logged_in()){
        wp_die('Maintenance, please come back soon.', 'Maintenance - please come back soon.', array('response' => '503'));
    }
}
add_action('get_header', 'wp_maintenance_mode');