$plugins_dir
插件目录变量
This is the directories where Smarty will look for the plugins that it
needs. Default is "plugins" under the SMARTY_DIR. If you supply a
relative path, Smarty will first look under the SMARTY_DIR, then
relative to the cwd (current working directory), then relative to each
entry in your PHP include path.
该变量定义Smarty寻找所需插件的目录。默认是在SMARTY目录下的“plugins”目录。如果提供了一个相对路径，Smarty将首先在SMARTY目录下寻找，然后到当前工作目录下寻找，继而到php包含路径中的每个路径中寻找。
Technical Note: For best performance, do not setup your plugins_dir to have to use the PHP include path. Use an absolute pathname, or a path relative to SMARTY_DIR or the cwd.
技巧：为了获取最好性能，不要将插件目录安装成必须使用php包含路径的境地。最好用一个绝对路径名字，或者是一个相对SMARTY目录或当前工作目录的路径。