url redirect

[eluser]Unknown[/eluser]
how to do these effect in codeigniter :
To visit url: /a/b , but must login in correctly. if not, redirect to /login/index . then logining, if correctly login. redirect to url /a/b again.
I tried many times, use rawurlencode() to let /a/b as part of request url , but show not found page. would you please give me one idea? thanks.