Something like this should do it...
$strParent_Directory = rtrim(dirname($_SERVER['REQUEST_URI']), '/');
$strParent_Directory = substr($strParent_Directory, 0, (strrpos($strParent_Directory, '/') + 1));
$strComplete_URL = 'http://' . $_SERVER['HTTP_HOST'] . $strParent_Directory . 'file.php';
header('Location: ' . $strComplete_URL);
... where "file.php" is the file you want to redirect to in the parent directory...