php静态文件回到304

<span style="color: #0000ff;">private</span> <span style="color: #0000ff;">function</span> _addEtag(<span style="color: #800080;">$file</span><span style="color: #000000;">) {    </span><span style="color: #800080;">$last_modified_time</span> = <span style="color: #008080;">filemtime</span>(<span style="color: #800080;">$file</span><span style="color: #000000;">);     </span><span style="color: #800080;">$etag</span> = <span style="color: #008080;">md5_file</span>(<span style="color: #800080;">$file</span><span style="color: #000000;">);    </span><span style="color: #008000;">//</span><span style="color: #008000;"> always send headers </span>    <span style="color: #008080;">header</span>("Last-Modified: ".<span style="color: #008080;">gmdate</span>("D, d M Y H:i:s", <span style="color: #800080;">$last_modified_time</span>)." GMT"<span style="color: #000000;">);     </span><span style="color: #008080;">header</span>("Etag: <span style="color: #800080;">$etag</span>"<span style="color: #000000;">);     </span><span style="color: #008000;">//</span><span style="color: #008000;"> exit if not modified</span>    <span style="color: #0000ff;">if</span> (@<span style="color: #008080;">strtotime</span>(<span style="color: #800080;">$_SERVER</span>['HTTP_IF_MODIFIED_SINCE']) == <span style="color: #800080;">$last_modified_time</span> ||<span style="color: #000000;">     @</span><span style="color: #008080;">trim</span>(<span style="color: #800080;">$_SERVER</span>['HTTP_IF_NONE_MATCH']) == <span style="color: #800080;">$etag</span><span style="color: #000000;">) {         </span><span style="color: #008080;">header</span>("HTTP/1.1 304 Not Modified"<span style="color: #000000;">);         </span><span style="color: #0000ff;">exit</span><span style="color: #000000;">;     }}</span>