$config_dir
配置目录变量
This is the directory used to store config files used in the
templates. Default is "./configs", meaning that it will look
for the configs directory in the same directory as the
executing php script.
该变量定义用于存放模板配置文件的目录，默认情况下，目录是：“./configs”，也就是说他将会在和php执行脚本相同的目录下寻找配置目录。
Technical Note: It is not recommended to put this directory under the web server document root.
技巧：不推荐把编译目录放在web服务器根目录下。