今天ytkah的客戶反饋說他的xml網站地圖有問題，提示Sitemap Error : XML declaration allowed only at the start of the document，這個很大的可能是wp-config.php或主題function.php文件中有多余的空格/空行，打開他的function.php看了一下，發現在最后加了一些定義
<?php
remove_action('wp_head', 'rsd_link');
remove_action('wp_head', 'wlwmanifest_link');
?>
這個是另起了一個<?php ?>，試着把它去掉，然后把remove定義放到前面的封裝里，保存上傳覆蓋，更新緩存，更新cdn，再訪問一下xml網站地圖不會出現前面的錯誤提示了。
參考資料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.