PHP Header with Location seem to redirect faster than using Refresh. Looks like Refresh will create the <meta> refresh tag and depends on the Browser to redirect. In FireFox, it will actually show a blank page before it redirects to destination page.
With Location option, it seems to redirect directly without showing the blank page.
header("Refresh:0;url=". $this->config->item('PF_HTTPS_ROOT') . '/member/signin/');
header('Location: ' . $this->config->item('PF_HTTPS_ROOT') . '/member/signin/', TRUE, 301);
Either ways, it will have a round trip between Server and Client's browser.