CURRENT_LANG:
the current language the user uses. Should be provided in the url.
CURRENT_ACTION:
the action provided in $_REQUEST['action']. Used to stay on the same page when changing the language.
TEMPLATE:
CURRENT_ACTION . ".html"; used to display the template corresponding to the current action in main.html.
get variables
index.php
action: the task that should be performed by php (e.g. template that should be loaded). However, templates may depend on other variables, for instance if the user is logged in
lang: specifies the language config file. As a result, the page is displayed in that language