WordPress Must-use plugin for HTTPS behind Varnish Proxy

varnish.php

<?php
/**
 * Plugin Name: HTTPS for wp-admin
 * Description: Forces SSL for wp-admin via X-Forwarded-Proto.
 *
 * https://wpxss.com/wp-config-php/how-to-set-up-wordpress-behind-a-secure-reverse-proxy-using-nginx/
 */

add_action('muplugins_loaded', function () {
    if ( isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https' ) {
        $_SERVER['HTTPS'] = 'on';
    }

    if ( ! defined('FORCE_SSL_ADMIN') ) {
        define('FORCE_SSL_ADMIN', true);
    }
});