If set to true, Smarty will respect the If-Modified-Since
header sent from the client. If the cached file timestamp has
not changed since the last visit, then a "304 Not Modified"
header will be sent instead of the content. This works only on
cached content without {insert} tags.
如果设置该变量为真,Smarty将分析客户端发送来的If-Modified-Since头信息.如果缓存文件时间戳自上次访问以来没有改变,则发送一个"304 Not Modified"头,而不是缓存文件内容.这种方式仅工作于没有{insert}标记的缓存内容.