Drupal7 General utility variables

When we create a new theme of Drupal, we should know the following variable to understand the files such as page.tpl.php and template.php.

- $base_path: The base URL path of the Drupal installation. At the very
  least, this will always default to /.
- $directory: The directory the template is located in, e.g. modules/system
  or themes/bartik.
- $is_front: TRUE if the current page is the front page.
- $logged_in: TRUE if the user is registered and signed in.
- $is_admin: TRUE if the user has permission to access administration pages.

Site identity:
- $front_page: The URL of the front page. Use this instead of $base_path,
  when linking to the front page. This includes the language domain or
  prefix.
- $logo: The path to the logo image, as defined in theme configuration.
- $site_name: The name of the site, empty when display has been disabled
  in theme settings.
- $site_slogan: The slogan of the site, empty when display has been disabled
  in theme settings.
- $hide_site_name: TRUE if the site name has been toggled off on the theme
  settings page. If hidden, the "element-invisible" class is added to make
  the site name visually hidden, but still accessible.
- $hide_site_slogan: TRUE if the site slogan has been toggled off on the
  theme settings page. If hidden, the "element-invisible" class is added to
  make the site slogan visually hidden, but still accessible.

Navigation:
- $main_menu (array): An array containing the Main menu links for the
  site, if they have been configured.
- $secondary_menu (array): An array containing the Secondary menu links for
  the site, if they have been configured.
- $breadcrumb: The breadcrumb trail for the current page.

Page content (in order of occurrence in the default page.tpl.php):
- $title_prefix (array): An array containing additional output populated by
  modules, intended to be displayed in front of the main title tag that
  appears in the template.
- $title: The page title, for use in the actual HTML content.
- $title_suffix (array): An array containing additional output populated by
  modules, intended to be displayed after the main title tag that appears in
  the template.
- $messages: HTML for status and error messages. Should be displayed
  prominently.
- $tabs (array): Tabs linking to any sub-pages beneath the current page
  (e.g., the view and edit tabs when displaying a node).
- $action_links (array): Actions local to the page, such as 'Add menu' on the
  menu administration interface.
- $feed_icons: A string of all feed icons for the current page.
- $node: The node object, if there is an automatically-loaded node
  associated with the page, and the node ID is the second argument
  in the page's path (e.g. node/12345 and node/12345/revisions, but not
  comment/reply/12345).