How enable “If-Modified-Since”

Hi, @tenkstars. Then I have a question. When I enable “If-Modified-Since” via .htaccess, the page code is still 200. Does the Lightspeed plugin affect the fact that the server cannot respond with ” 304 Not Modified”?

It seems to me that this is an important point for everyone who uses “Last-Modified” and “If-Modified-Since”. It is necessary that the page response is 304 for unchanged pages, but with the Litespeed plugin, the server response is always 200. Google also recommends using the “Last-Modified” and “If-Modified-Since” headers. Therefore, I would like to understand why the plugin removes “If-Modified-Since”, what changes the server response code and how to fix it?

Sorry, I use a translator to communicate. Perhaps you didn’t understand me. I will try to explain the situation.

I need the server response to be 304 if the resource has not been changed.
If we checking the site for the Last-Modified header on the resource https://last-modified.com/en, it informs us that the site gave the time of the last modification, but did not respond to “If-Modified-Since”.

Since it is not yet possible to enable the “If-Modified-Since” header in the Litespeed plugin, I added the following code manually:

<?php
$LastModified_unix = strtotime(date("D, d M Y H:i:s", filectime($_SERVER['SCRIPT_FILENAME'])));
$LastModified = gmdate("D, d M Y H:i:s \G\M\T", $LastModified_unix);
$IfModifiedSince = false;

if (isset($_ENV['HTTP_IF_MODIFIED_SINCE']))
   $IfModifiedSince = strtotime(substr ($_ENV['HTTP_IF_MODIFIED_SINCE'], 5));

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']))
   $IfModifiedSince = strtotime(substr ($_SERVER['HTTP_IF_MODIFIED_SINCE'], 5));

if ($IfModifiedSince && $IfModifiedSince >= $LastModified_unix) {
   header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
   exit;
}
header('Last-Modified: '. $LastModified);
?>

After that, we test the site again, but the result has not changed. The pages will still be without “If-Modified-Since” and the server response code will be 200, despite the fact that the time of the Last-Modified has not changed.

This is probably because the “Last-Modified” header on the page is generated by the Litespeed plugin, and not by the server, so the server cannot give the “If-Modified-Since” header.

Yes, by default, WP does not include various headers. So I added the above code, which adds the If-Modified-Since and Last-Modified headers. Before switching to the server with Litespeed, I used the “Cache Enabler” plugin, and it automatically added these headers. It would be great if it was possible and Litespeed, as this speeds up the indexing of new pages, and also reduces the load on the server because Google, Yandex, Bing will get a response 304 unmodified pages and they do not need to re-scan the page.

Before writing to you, I turned to my hosting with a question about how to enable “If-Modified-Since”. They provided me with this code that needs to be inserted into header.php before the html tag.

<?php
$LastModified_unix = strtotime(date("D, d M Y H:i:s", filectime($_SERVER['SCRIPT_FILENAME'])));
$LastModified = gmdate("D, d M Y H:i:s \G\M\T", $LastModified_unix);
$IfModifiedSince = false;

if (isset($_ENV['HTTP_IF_MODIFIED_SINCE']))
   $IfModifiedSince = strtotime(substr ($_ENV['HTTP_IF_MODIFIED_SINCE'], 5));

if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']))
   $IfModifiedSince = strtotime(substr ($_SERVER['HTTP_IF_MODIFIED_SINCE'], 5));

if ($IfModifiedSince && $IfModifiedSince >= $LastModified_unix) {
   header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
   exit;
}
header('Last-Modified: '. $LastModified);
?>

In addition, there is another way, this code must be inserted into the file .htaccess.

RewriteEngine on
RewriteRule .* - [E=HTTP_IF_MODIFIED_SINCE:%{HTTP:If-Modified-Since}]
RewriteRule .* - [E=HTTP_IF_NONE_MATCH:%{HTTP:If-None-Match}]

When I told them that this method does not work for me, they said that they installed a clean WP on their test site, inserted 1 version of the code and everything worked for them. After that, they advised me to contact the developers.

You can test both methods. Also for testing, please pay attention to the plugin Cache-Enabler, it has support for 304 Not Modified.