<form class="form" role="form" method="POST" action="<?php echo base_url() . 'index/login' ?>" accept-charset="UTF-8">
    <?php
        echo validation_errors(); // Mensajes de error
    ?>
    <div class="form-group">
        <label class="sr-only" for="tbx_username">Nombre de usuario</label>
        <input type="text" class="form-control" name="tbx_username" id="tbx_username" placeholder="Nombre de usuario" />
    </div>
    <div class="form-group">
        <label class="sr-only" for="tbx_password">Contraseña</label>
        <input type="password" class="form-control" name="tbx_password" id="tbx_password" placeholder="Contraseña" />
    </div>

    <div class="form-group">
        <button type="submit" class="btn btn-primary btn-block">Iniciar Sesión</button>
    </div>
</form>

public function login()
{
    if($this->input->post())
    {
        $this->form_validation->set_rules('tbx_username', 'Usuario', 'required');
        $this->form_validation->set_rules('tbx_password', 'Contraseña', 'required');

        if($this->form_validation->run() == TRUE)
        {
            $username = $this->input->post('tbx_username');
            $password = $this->input->post('tbx_password');

            if($username == 'admin' && $password == '12345')
            {
                $this->session->set_userdata('logged_in', TRUE);
                $this->session->set_userdata('username', $username);
            }
        }
        redirect('/index', 'refresh');
    }
}