Custom $_GET vars in wordpress entry with permalink

Hello I have something like this in the url:
http://myblog.com/category/subcategory/postname/?var=true

Using it on functions.php:

function add_query_vars($vars) {
   $vars[] = "var";
   return $vars;
}
add_filter('query_vars', 'add_query_vars');

But I need it:
http://myblog.com/category/subcategory/postname/true

How can get it?