[PHP] NULL Apache environment variables

[Narvaez, Teresa]

Hello, 
        I'm running PHP version 4.1.1.  When I invoke this function, <?php
phpinfo()?>, all variables are set.  However, when I try to retrieve a value
of a certain variable like this: 

      <?php echo $HTTP_USER_AGENT; ?>  

It returns NULL.  Is there a missing configuration on apache or php? 

Thanks, 
-Teresa