The function defined() always return false within a function or class,
although the value of the constant is still available.

the code :

<?php
define(NAME, "value");
function f() {
     if (defined(NOM)) echo "NOM is defined";
     echo NOM;
}
f();
?>

will print "value" only. Currently tested with EasyPHP running with a
Windows XP computer.

Sorry, but your problem does not imply a bug in PHP itself.  For a
list of more appropriate places to ask for help using PHP, please
visit http://www.php.net/support.php as this bug system is not the
appropriate forum for asking support questions. 

Thank you for your interest in PHP.