$_SERVER[‘HTTP_IF_MODIFIED_SINCE’] отсутствует

В случае если, $_SERVER[‘HTTP_IF_MODIFIED_SINCE’] отсутствует:
.htaccess

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

php:

session_cache_limiter('private_no_expire');
session_start();