smarty路径问题

虚拟主机www.xxx.cn下：---+smarty----+Smarty.class.php
                            |
                            +www-----------+smarty.inc.php
                                           |
                                           +index.php   [require("smarty.inc.php")]
------------------------------------------------------------------
则：smarty.inc.php中是这样写的： define("SMARTY_DIR","./smarty/");
 define("WWW",SMARTY_DIR."/www/");
 require_once(SMARTY_DIR."Smarty.class.php");
但是 报错了：Warning: require_once(..Smarty.class.php) [function.require-once]: failed to open stream: No such file or directory in /home/acruxcn/public_html/zxs/smarty/www/smarty.inc.php on line 4
路径出错，应该怎么改正呢？