This is the name of the directory where template caches are stored. By default this is "./cache", meaning that it will look for the cache directory in the same directory as the executing php script. You can also use your own custom cache handler function to control cache files, which will ignore this setting.
此變數的內容為樣版快取儲存的檔案路徑，預設情況下為 "./cache"，代表的意 義為執行 php 程式檔相同目錄下的 caches 子目錄。你也可以使用你自己的客制化快取處理函 式以控制快取檔案，不過如此一來將會忽略 $cache_dir 的設定。
Technical Note: This setting must be either a relative or absolute path. include_path is not used for writing files.
技術提醒： 此變數內的值必須是絕對路徑或是相對路徑。include_path 內不寫入檔案。
Technical Note: It is not recommended to put this directory under the web server document root.
技術提醒： 不建議將此變數的值設定於網頁伺服器的根目錄之下。