php防止表单重复提交

public function index(){
        $_SESSION['token'] = md5(microtime(true));
        $this->display();
    }

<tr>
                    <td>
                        <input type="hidden" name="token" value="{$Think.session.token}">
                    </td>
                    <td><input type="submit" class="submit btn" value="立即注册"></td>
                </tr>

$token = I('post.token');
        if ($token !== session('token')) {
            $this->error('对不起，令牌验证失败！');
        }else{
            session('token',null);
        }

<form >