Rewrite a single's permalink

rewrite-permalinks.php

<?php
/**
 * Linking for "Ghost" post types
 *
 * @param $url
 * @param $post
 * @param bool $leavename
 *
 * @return mixed
 */
if (!function_exists('edit_news_permalink')) {
    add_filter('post_link', 'edit_news_permalink', 10, 3);
    add_filter('post_type_link', 'edit_news_permalink', 10, 3);

    function edit_news_permalink($url, $post, $leavename = false)
    {
        if ($post->post_type == 'news' && get_field('is_external', $post)) {
            $external_url = get_field('external_url', $post);

            if (!empty($external_url)) {
                $url = $external_url;
            }
        }

        return $url;
    }
}

/**
 * Redirecting for the News single template
 */
if (!function_exists('redirect_external_content')) {
    add_action('template_redirect', 'redirect_external_content');

    function redirect_external_content()
    {
        if (is_singular('news')) {
            global $post;

            if ($post->post_type == 'news' && get_field('is_external', $post)) {
                $external_url = get_field('external_url', $post);

                if (!empty($external_url)) {
                    wp_redirect(esc_url($external_url), 301);
                    exit;
                }
            }
        }
    }
}