This is the name of the default template directory. If you do
not supply a resource type when including files, they will be
found here. By default this is "./templates", meaning that it
will look for the templates directory in the same directory as
the executing php script.
该变量定义默认模板目录的名字。当包含文件时，如果不提供一个源类型（即源地址），那么将会到模板目录中寻找。默认情况下，目录是：“./templates”，也就是说他将会在和php执行脚本相同的目录下寻找模板目录。
Technical Note: It is not recommended to put this directory under the web server document root.
技巧注意：不推荐把模板目录放在web服务器根目录下。