php静态文件返回304，_PHP教程

<span>private</span> <span>function</span> _addEtag(<span>$file</span><span>) {
    </span><span>$last_modified_time</span> = <span>filemtime</span>(<span>$file</span><span>); 
    </span><span>$etag</span> = <span>md5_file</span>(<span>$file</span><span>);
    </span><span>//</span><span> always send headers </span>
    <span>header</span>("Last-Modified: ".<span>gmdate</span>("D, d M Y H:i:s", <span>$last_modified_time</span>)." GMT"<span>); 
    </span><span>header</span>("Etag: <span>$etag</span>"<span>); 
    </span><span>//</span><span> exit if not modified</span>
    <span>if</span> (@<span>strtotime</span>(<span>$_SERVER</span>['HTTP_IF_MODIFIED_SINCE']) == <span>$last_modified_time</span> ||<span> 
    @</span><span>trim</span>(<span>$_SERVER</span>['HTTP_IF_NONE_MATCH']) == <span>$etag</span><span>) { 
        </span><span>header</span>("HTTP/1.1 304 Not Modified"<span>); 
        </span><span>exit</span><span>; 
    }
}</span>