今天ytkah的客户反馈说他的xml网站地图有问题，提示Sitemap Error : XML declaration allowed only at the start of the document，这个很大的可能是wp-config.php或主题function.php文件中有多余的空格/空行，打开他的function.php看了一下，发现在最后加了一些定义
<?php
remove_action('wp_head', 'rsd_link');
remove_action('wp_head', 'wlwmanifest_link');
?>
这个是另起了一个<?php ?>，试着把它去掉，然后把remove定义放到前面的封装里，保存上传覆盖，更新缓存，更新cdn，再访问一下xml网站地图不会出现前面的错误提示了。
参考资料However, the cause most often is an empty line at the beginning (before the <?php line) or end of the wp-config.php or functions.php file. If there is no empty line in these files, we highly recommend running a conflict check to identify what outputs the empty whitespace. You may also need to clear your caching from your plugins/theme/CDN such as Cloudflare/server caching etc.