Callback is failed in Form Validation

[eluser]CyrusTC[/eluser]
Hi Everyone,

I am new to codeigniter and i just wanna ask about a failure of executing the callback function in the form validation

Code:

$validation_rule = array(
        array(
          'field' => 'username',
          'label' => 'Username',
          'rules' => 'required|callback_unique_username'
        ),
        array(
          'field' => 'email',
          'label' => 'Email',
          'rules' => 'required|valid_email'
        ),
        array(
          'field' => 'password',
          'label' => 'Password',
          'rules' => 'required|callback_same_password['.$this->CI->input->post('confpassword').']',
        ),
        array(
          'field' => 'confpassword',
          'label' => 'Confirm Password',
          'rules' => 'required|callback_same_password['.$this->CI->input->post('confpassword').']',
        )
      );

In Fact, the above are validation rules of a form and you are able to see i put callback_unique_username in the rule of username, where callback_unique_username is

Code:

function unique_username($str)
    {
     $this->CI->form_validation->set_message('unique_username', 'The %s field can not be the word "test"');
     return FALSE;
    }

This Callback validation function is just a test function, in which it gives a error message anyway. However, the error message isn't shown in validation_errors().
Could any guys tell me what's going wrong? Thanks.