If you use Apache's redirection features for custom error pages or whatever, the following Apache's REDIRECT variables are also available in $_SERVER:
$_SERVER['REDIRECT_UNIQUE_ID]'
$_SERVER['REDIRECT_SCRIPT_URL]'
$_SERVER['REDIRECT_SCRIPT_URI]'
$_SERVER['REDIRECT_SITE_ROOT]'
$_SERVER['REDIRECT_SITE_HTMLROOT]'
$_SERVER['REDIRECT_SITE_CGIROOT]'
$_SERVER['REDIRECT_STATUS]'
$_SERVER['REDIRECT_QUERY_STRING]'
$_SERVER['REDIRECT_URL]'
I'm not sure if this is a complete list though