$compile_dir
编译目录变量
This is the name of the directory where compiled templates are
located. By default this is "./templates_c", meaning that it
will look for the compile directory in the same directory as
the executing php script.
该变量定位编译模板的目录名字。默认情况下，目录是：“./templates_c”，也就是说他将会在和php执行脚本相同的目录下寻找编译目录。
Technical Note: This setting must be either a relative or absolute path. include_path is not used for writing files.
技巧：该设置必须是一个相对或绝对路径。包含路径不用于写文件。
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。