What's get_header? It's not a PHP function, and you've not defined it in the code you've posted, so PHP doesn't know what you're trying to do. If you want to include 'header.php' in your file, use include, require, include_once or require_once.

include simply inserts the contents of another file into the script. require does the same thing, but it also prints an error and kills the script if it can't find the file. The '_once' versions also make sure the file is only included once in the script. So if you want to use require, you would write require 'header.php';.

You'll probably get another error when you get to the get_footer(); call at the bottom of the script; just do the same thing as above, replacing 'footer.php' for 'header.php'.