Addendum: The code

<pre>
<?php
  print_r(apache_request_headers());
?>
</pre>

gives me

 Array
 (
    [X-Powered-By] => PHP/5.1.6
 )

But the code
<pre>
<?php
  ob_end_flush();
  print_r(apache_request_headers());
?>
</pre>

Results in

 Array
 (
    [Content-Location] => phpinfo.de.php
    [Vary] => negotiate
    [TCN] => choice
    [X-Powered-By] => PHP/5.1.6
    [Keep-Alive] => timeout=15, max=96
    [Connection] => Keep-Alive
    [Transfer-Encoding] => chunked
    [Content-Type] => text/html
    [Content-Language] => de
 )