Description:
------------
When calling the header() function in a function registred with register_shutdown_function(), PHP issues a warning as it there had been an output before.

Reproduce code:
---------------
<?php
 register_shutdown_function('destructor');

 function destructor()
 {
  header("HTTP/1.1 200 OK");
 }
?>

Expected result:
----------------
The header correctly inserted in the output.

Actual result:
--------------
Warning: Cannot modify header information - headers already sent in g:\[...]\bugtest.php on line 6