constants STDOUT, STDERR, STDIN not working in 5.2.x?

Hello,

I found different code examples like this, which use the file handle 
STDERR just like this:

<?php
fwrite(STDERR, "hello\n");
?>

Also, the PHP documentation of input/output streams 
(http://php.net/manual/de/wrappers.php.php) says:

"It is recommended that you simply use the constants STDIN, STDOUT  and 
STDERR instead of manually opening streams using these wrappers."

I don't want to use the "php://stderr" wrapper, because this just 
creates a duplicate of the original STDERR handler and if I'm closing 
"php://stderr", the original STDERR still would exist.

When I'm using this code, I only get:

<b>Notice</b>:  Use of undefined constant STDERR - assumed 'STDERR' in 
<b>/test.php</b> on line <b>4</b><br />
<br />
<b>Warning</b>:  fwrite(): supplied argument is not a valid stream 
resource in <b>/test.php</b> on line <b>4</b><br />

How can I access the original STDERR handle? The constant should be 
there, but does not exist.

regards
Marten