Call internal pages with drupal_http_request
Here is what I'm using in a similar situation, with $base_url as Baysaa mentioned above.

$path_to_file = '/path/to/feed.xml';
$base_url = $GLOBALS['base_url']; // Not sure if this line is necessary.
$file_over_http = $base_url . $path_to_file;
$http_result = drupal_http_request($file_over_http);