Do I have to activate/set something on the server in order to use these?
PHP Code:
header('Last-Modified: '.gmdate('D, d M Y H:i:s', $last_mod_header).' GMT');
header('If-Modified-Since: '.gmdate('D, d M Y H:i:s', $last_mod_header).' GMT');
header('Expires: '.gmdate('D, d M Y H:i:s', time() + 604800).' GMT');
Can they be dangerous to the update frequency of search engine ?