bottom.php和bthemeshow.php

bottom.php中右侧显示最新帖子功能中有个<a href="bthemeshow.php?$theme_id=<?php echo
$arr['id'];?>" target="_blank"><?php echo
$arr['title'];?></a>链接，传值$theme_id=...到bthemeshow.php中，值传不过去，提示Undefined
variable: theme_id in D:\xampp\htdocs\forum\bthemeshow.php on line 5