PHP 函数 Header Register Callback

<?php
   header('Content-Type: text/plain');
   header('X-Test: f1');

   function f1() {
      foreach (headers_list() as $header) {

         if (strpos($header, 'X-Powered-By:') !== false) {
            header_remove('X-Powered-By');
         }
         header_remove('X-Test');
      }
   }

   $result = header_register_callback('f1');

   echo "codingdict";
?>