Simple Maintenance Mode redirect

ti-maintenance-mode.php

<?php
/**
 * Plugin Name: [TI] Maintenance Mode
 * Plugin URI: https://www.thoughtsandideas.uk
 * Description:
 * Author: Michael Bragg <michael.bragg@thoughtsandideas.uk>
 * Author URI: https://www.thoughtsandideas.uk
 * Version: 1.0.0
 * License: GPLv2
 */
function ti_maintenance_mode() {

  // Set `define( 'TI_MAINTENANCE_MODE', true)` in your `wp-config.php` file.
	if ( ! defined( 'TI_MAINTENANCE_MODE' ) || false == TI_MAINTENANCE_MODE ) {
		return;
	}

	// Use `list_users` as check to allow Shop Managers access.
	if( ! is_user_logged_in() && ! current_user_can( 'list_users' ) ) {

		wp_redirect(
      // Path to holding page file.
			get_option( 'home' ) . '/maintenance.html',
			307
		); exit;

	}

}

add_action(
	'get_header',
	'ti_maintenance_mode'
);