data_global.php is only a cache file which contains preparsed data from the scripts.
You declare the template variables in other places. If you want to be able to use the variable on all pages, you declare it in includes/functions.php, in the page_header() function. If you want to use it only on a specific page, declare it in that page's PHP file.
Look for $template->assign_vars(. This might also be a good resource to start with: http://olympuswiki.naderman.de/Template_Syntax