How to make Remote Function work in codeigniter framework?

[eluser]jiahui[/eluser]
I didn't use any $_GET array in my controller site

Code:

function check_user_availability()
        {
            $request = trim(strtolower($_REQUEST['username']));
            //sleep(2);
            usleep(150000);
            $users = array('asdf', 'Peter', 'Peter2', 'George');
            $valid = 'true';
            foreach($users as $user) {
                if( strtolower($user) == $request )
                    $valid = 'false';
            }
            echo $valid;
                    }

but I am using direct URL. There has nothing response from CI return.
Could you help? Appreciate if you can help me solve it.