What am I doing wrong?

I am on the basic build a php website here https://teamtreehouse.com/library/build-a-basic-php-website/building-a-media-library-in-php/working-with-get-variables

I am following along with the video. As far as I can tell I have done exactly as instructed but when adding the _GET variable I am getting the following errors.

Notice: Use of undefined constant _GET - assumed '_GET' in /home/treehouse/workspace/catalog.php on line 3

Warning: Illegal string offset 'cat' in /home/treehouse/workspace/catalog.php on line 3

Notice: Use of undefined constant _GET - assumed '_GET' in /home/treehouse/workspace/catalog.php on line 5

Warning: Illegal string offset 'cat' in /home/treehouse/workspace/catalog.php on line 5

Notice: Use of undefined constant _GET - assumed '_GET' in /home/treehouse/workspace/catalog.php on line 7

Warning: Illegal string offset 'cat' in /home/treehouse/workspace/catalog.php on line 7

Here is my code for the page

<?php $pagetitle = "Full Catalog"; if (_GET["cat"] == "books") { $pagetitle = "Books"; } else if (_GET["cat"] == "movies") { $pagetitle = "movies"; } else if (_GET["cat"] == "music") { $pagetitle = "music"; }

include("inc/header.php"); ?> <div class="section page"></div> <h1><?php echo $pagetitle; ?></h1>

<?php include("inc/footer.php"); ?>

I am a noob trying to get this down please help!!!