Why can't I use trim() in CodeIgniter 3.0?

Because of the version of apache and php, I cannot use CI's form_validation library and filter function, I have to write some native code to filter the special character. However, I cannot use trim() on model or controller, even though I add $this->load->library('string'). Anyone knows how to solve it? Or anyone give me some suggestion to do it without using CI's form validation. Thank you!