我扩展了 CI 表单验证库,如下所示:
class MY_Form_validation extends CI_Form_validation {
    function __construct($config = array())
    {
        parent::__construct($config);
    }
    function check_first_char($str) 
    {
        $CI =& get_instance();
        $first_char = substr($str, 0, 1);
        if ($first_char != 'P' || $first_char != 'S') 
        {
            $CI->form_validation->set_message('check_first_char', 'The %s field must begin with P or S!');
            return FALSE;
        }
        else {
            return TRUE;
        }
    }
并像下面这样调用它:
$this->form_validation->set_rules('sponsor_id', 'Sponsor ID', 'trim|required|exact_length[7]|check_first_char');
但它不起作用。我做错了什么?