CSRF会话令牌

我试图在表单上实现一个简单的CSRF保护，但我做不好。如果有人能指出我做错了什么，我将不胜感激

错误：每次我提交表单时，我都会收到"Invalid Submission2"，因为令牌会更改（在表单提交后），因为是在同一页面上生成的？

编辑--我忘了提一下，另一个文件（config.php）已经有session_start（）了。

<?php
class Module_Post extends Base_Module {
    public function start()
    {
        requireLogin();
        if (isset($_GET['act'])) {
            switch($_GET['act']) {
                case 'text':
                    $this->text();
                    break;
                case 'image':
                    $this->image();
                    break;
                default:
                    $this->text();
                    break;
            }
        } else {
            $this->text();
        }
    }
    private function text()
    {
        // Generate random unique token to prevent cross-site request forgery (CSRF).
        if(empty($_SESSION['form_token'])) 
        {
            $form_token = md5(uniqid(rand(), TRUE));
            $_SESSION['form_token'] = $form_token;
        }
        if(isset($_POST['submit'])) 
        {
            // Clean the content from cross-site scripting (xss)
            $content = trim($_POST['content']);
            $content = Xss::clean($content);
            // Validate that the form token
            if(!isset($_POST['form_token'], $_SESSION['form_token'])) {
                $err = 'Invalid Submission';
            } elseif ($_POST['form_token'] != $_SESSION['form_token']) {
                $err = 'Invalid Submission2';
            } elseif (strlen($content) < 10) {
                $err = 'Your content contains too few characters.';
            }
            if(isset($err)) {
                unset( $_SESSION['form_token']);
                $this->setMessage($err, 'FAIL');
                header('Location: index.php?mod=post');
                exit;
            }

            // Insert database data here, then redirect
            $this->setMessage('Your post was published successfully.', 'GOOD');
            header('Location: index.php');
            exit;
        }
        $this->tpl->assign('form_token', $form_token);
        $this->tpl->display('new/text.tpl');
    }
}
?>

HTML（text.tpl文件）

<form method='post' enctype='multipart/form-data' action='#'>
  <fieldset>
    <textarea rows="8" id="new_post" name="content" class="input-block-level"></textarea>
    <input type="hidden" name="form_token" value="{$form_token}" />
    <button type="submit" name="submit" class="btn btn-info pull-left">Create Post</button>
  </fieldset>
</form>