TwitterOAuth

    public function twitter()
    {
        // The TwitterOAuth instance

        $twitteroauth = new TwitterOAuth($this->config->item('key', 'twitter'), $this->config->item('secret', 'twitter'));
        // Requesting authentication tokens, the parameter is the URL we will be redirected to
        $request_token = $twitteroauth->getRequestToken( site_url() . 'auth/twitter_oauth' );

        // Saving them into the session
        $array = array(
            'oauth_token' => $request_token['oauth_token'],
            'oauth_token_secret' => $request_token['oauth_token_secret']
        );

        $this->session->set_userdata( $array );

        // If everything goes well..
        if($twitteroauth->http_code==200){
            // Let's generate the URL and redirect
            $url = $twitteroauth->getAuthorizeURL($request_token['oauth_token']);
            header('Location: '. $url);
        } else {
            // It's a bad idea to kill the script, but we've got to know when there's an error.
            die('Something wrong happened.');
        }

    }