Error with blank User-Agent: header

Line 12 of the file advanced-cache.php refers to:

$_SERVER[‘HTTP_USER_AGENT’].

As of PHP 8, if HTTP_USER_AGENT is not set, this will produce a warning. While it is reasonable to expect this to be set, it isn’t always, particularly by some RSS readers. To avoid the warning, this should be changed to:

$_SERVER[‘HTTP_USER_AGENT’] ?? “”