Call to undefined function I()

<?php
class LoginAction extends Action {

//登录页面视图
public function index(){
$this->display();
}

//登录表单
public function login(){
//if(!IS_POST) halt('页面不存在');
echo $pwd = I('password','','md5');
// echo session('verify');
// if($_POST['code'] != session('verify')) $this->error("验证码错误");
// $db = M('user');
// $user = $db->where(array('username'=>I('username')))->find();
// p($user);

}

public function verify(){
import('Class.Image',APP_PATH);
Image::verify();
}


}


?>