$currentHandler
$currentHandler : \SessionHandlerInterface&\SessionUpdateTimestampHandlerInterface
Migrating session handler for migrating from one handler to another. It reads from the current handler and writes both the current and new ones.
It ignores errors from the new handler.
$currentHandler : \SessionHandlerInterface&\SessionUpdateTimestampHandlerInterface
$writeOnlyHandler : \SessionHandlerInterface&\SessionUpdateTimestampHandlerInterface