The file with the code I gave you should just be called ".htaccess" (with the dot at the beginning, and without the quotes)
You save this .htaccess file in the same folder as Pages.php
Pages.php should look like this:
Once you've saved the .htaccess file where you should, go to your web browser, and browse to Pages.xml (Pages.xml doesn't exist in reality, but the htaccess file will tell apache to use Pages.php when Pages.xml is being requested.)
There should be no file called Pages.xml in your folder. Just the .htaccess file and Pages.php