php验证字符串长度问题

C:UsersAdministrator>php -r "echo strlen('你好')";
4
C:UsersAdministrator>php -r "echo mb_strlen('你好')";
4
C:UsersAdministrator>php -r "echo mb_strlen('你好','utf-8')";
3
C:UsersAdministrator>php -r "echo mb_strlen('你好','gbk')";
2
C:UsersAdministrator>php -r "echo mb_strlen('你好','gb2312')";
2
C:UsersAdministrator>php -r "echo mb_strlen('你好','i')";