filters\HttpCache $lastModified

A PHP callback that returns the UNIX timestamp of the last modification time. The callback's signature should be:

function ($action, $params)

where $action is the yii\base\Action object that this filter is currently handling; $params takes the value of $params. The callback should return a UNIX timestamp.

See also http://tools.ietf.org/html/rfc7232#section-2.2.