Create new helper file and write the following function

if ( ! function_exists('flash'))
{
    function flash($type, $msg, $bold = '')
    {
        $ci=& get_instance();
        //type:success, info, warning, danger
        if($bold != ''){
            $bold = '<strong>'.$bold.'!</strong> ';
        }
        $flash = $ci->session->userdata('flash')?$ci->session->userdata('flash'):array();

        $flash[] = '
        <div class="alert alert-'.$type.' alert-dismissible" role="alert">
          <button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">&times;</span></button>
          '.$bold.$msg.'
        </div>
        ';

        $ci->session->set_userdata('flash',$flash);
    }   
}