1)
when trying something like this:

error_reporting(63);
define("CONSTANT", "Hello dfdfdffd.");
define("CONSTANT", "Hello world.");

it would be nice to get a warning when the second define takes no effect.

2)
in the doc-file (.../lang-const.html#CONSTANTS, Date: 1998-12-28, Release: 3.0.7-dev) I found:

<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
undefine ("CONSTANT");
?>

but i results in:

Hello world.<br>
<b>Fatal error</b>:  Call to unsupported or undefined function undefine() in <b>...</b> on line <b>...</b><br>

and in the actuell doc-file I miss it!?

3)
is it possible to test a constant wether it is set like:

define("CONSTANT", "Hello world.");
if(isset(CONSTANT))
    ......