PHP Newbie Needs a Hand

Hi,
I just installed PHP so I'm a complete newbie to this. I have a problem already, I'm not sure if it's normal or not but perhaps one of ye guys can help me out.

I'm trying out the Introductory Tutorial on PHP.net here --> http://www.php.net/tut.php and I've fallen at the second hurdle.

I want to write the PHP script that produces the user's browser type. The tutorial at PHP.net says I should use this:
[PHP]<?php echo $HTTP_USER_AGENT; ?>[/PHP]
It seems to make sense but it doesn't work. I just get a blank page.

So, in an effort to find out what the problem was, I went and made a script containing this code:
[PHP]<?php phpinfo(); ?>[/PHP]
This should have given me loads of info about PHP, which it did. I went looking for the variable $HTTP_USER_AGENT on that page but couldn't find it. Instead I found the variable $_SERVER["HTTP_USER_AGENT"] and used this in the script that I wanted to show the browser type.

So now my script looks like this:
[PHP]<?php echo $_SERVER["HTTP_USER_AGENT"]; ?>[/PHP]

That did the job. But is this normal? Did they change the names of the variables and, therefor, is the tutorial I'm reading out of date or something?