验证未停止数据，函数继续插入

我有一个快速问题：

在下面的函数中，我想要if the username is taken return the error return false but if it doesn't exist return true and continue，我认为我的T/F顺序不正确。

public function username_check($username)
    {
        if($username == TRUE)
        {
            $this->form_validation->set_message('username_check','Sorry the username %s is already taken!');
            return true;
        }else{
            return FALSE;
        }
    }

更新：

我的toms代码工作正常，但我现在有一个问题，回调似乎不起作用，它只是不断地将用户添加到数据库中，即使它已经存在！

视图：

<h1><?php echo $companyName; echo nbs(1);?> - <?php echo $pageTitle; ?></h1>

<?php
if($success == TRUE) {
echo '<section id = "validation">Page Updated</section>';   
}
?>
    <p>Error: <?php echo validation_errors();?></p>
    <div class="formContent">
        <form action="createUser" method="post">
            <fieldset class="control-group">
                <label for="userName">User Name:</label><input type="text" id="userName" name="userName"  value="<?php echo set_value('userName'); ?>" placeholder="User Name">
                <label for="userPassword">User Password:</label><input type="password" id="userPassword" name="userPassword" placeholder="User Password">
                <label for="userFirstName">First Name:</label><input type="text" id="userFirstName" name="userFirstName" value="<?php echo set_value('userFirstName'); ?>" placeholder="First Name">
                <label for="userLastName">Last Name:</label><input type="text" id="userLastName" name="userLastName" value="<?php echo set_value('userLastName'); ?>" placeholder="Last Name">
                <label for="userEmail">E-Mail:</label> <input type="text" id="userEmail" name="userEmail"  value="<?php echo set_value('userEmail'); ?>" placeholder="Admin E-mail">
                <label for="userGroup"> User Group:</label>
                    <select name="userGroup" id="userGroup">
                        <option value="select"<?=(set_value('userGroup')=='select')?' selected="selected" ':''?>>Please Select</option>
                        <option value="admin" <?=(set_value('userGroup')=='admin')?' selected="selected" ':''?>>Admin</option>
                        <option value="user" <?=(set_value('userGroup')=='user')?' selected="selected" ':''?>>User</option>
                    </select>
                <label for="userActive"> User Active:</label>
                        <select name="userActive" id="userActive">
                            <option value="select" <?=(set_value('userActive')=='select')?' selected="selected" ':''?>>Please Select</option>
                            <option value="yes" <?=(set_value('userActive')=='yes')?' selected="selected" ':''?>>Yes</option>
                            <option value="no" <?=(set_value('userActive')=='no')?' selected="selected" ':''?>>No</option>
                        </select>
                <div>
                    <button type="submit" class="btn-primary">Create</button>
                </div>
                </fieldset>
        </form>
    </div>

控制器：

public function index()
    {
        //Form Validation prep making sure its all clean
        $this->form_validation->set_rules('userName', 'User Name', 'trim|required|callback_username_check|xss_clean');
        $this->form_validation->set_rules('userPassword', 'User Password', 'trim|required|xss_clean|sha1');
        $this->form_validation->set_rules('userFirstName', 'First Name', 'trim|required|xss_clean');
        $this->form_validation->set_rules('userLastName', 'Last Name', 'trim|required|xss_clean');
        $this->form_validation->set_rules('userEmail', 'E-Mail', 'trim|required|xss_clean');
        $this->form_validation->set_rules('userGroup', 'User Group', 'trim|required|xss_clean');
        $this->form_validation->set_rules('userActive', 'User Active', 'trim|required|xss_clean');
        //If form validation fails load previous page with errors else do the job and insert data into db
        if($this->form_validation->run() == FALSE)
        {
            $data['success'] = "";
        }else{
            $username = $this->input->post('userName');
            $password = $this->input->post('userPassword');
            $firstname = $this->input->post('userFirstName');
            $lastname = $this->input->post('userLastName');
            $email = $this->input->post('userEmail');
            $group = $this->input->post('userGroup');
            $active = $this->input->post('userActive');
            $passwordHash = $this->encrypt->sha1($password); // Lets encrypt the password why sha1?  MD5 is for tossers
            // If the data is correct follow through with db insert
            if($this->users_model->createUser($username,$passwordHash,$firstname,$lastname,$email,$group,$active))
            {
                $data['success'] = TRUE;
            }
        }
        $data['companyName'] = $this->core_model->companyName();
        $data['pageTitle'] = "Create User";
        $this->load->view('admin/assets/header', $data);
        $this->load->view('admin/createUser', $data);
        $this->load->view('admin/assets/footer');
    }
    public function username_check($username)
    {
        if($this->users_model->username_taken($username))
        {
            $this->form_validation->set_message('username_check', 'Sorry the username %s is taken!');
            return TRUE;
        }else{
            return FALSE;
        }
    }

型号：

function createUser($username = NULL ,$passwordHash = NULL ,$firstname = NULL ,$lastname = NULL ,$email = NULL,$group = NULL ,$active = NULL)
{
    $data = array('userName' => $username, 'userFirstName' => $firstname, 'userLastName' => $lastname, 'userEmail' => $email, 'userPassword' => sha1($passwordHash), 'userGroup' => $group, 'userActive' => $active);
$this->db->insert('users',$data);
return TRUE;
}
function username_taken($username)
{
    $this->db->select('userName');
    $this->db->from('users');
    $this->db->where('userName', $username);
    $query = $this->db->get();
    if($query->num_rows() > 0)
    {
        return TRUE;
    }else{
        return FALSE;
    }