Description:
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There is no description of SessionUpdateTimestampHandlerInterface (introduced in PHP 7.0.0) and its respective non-oop session-set-save-handler arguments in the docs.

http://php.net/manual/en/class.sessionupdatetimestamphandlerinterface.php
and 
http://php.net/manual/en/function.session-set-save-handler.php

For the context of the updateTimestamp() function I would like to see
- What is the difference between SessionHandlerInterface::write and SessionUpdateTimestampHandlerInterface::updateTimestamp functions?
- How should differ the code implementing these two functions?
- How is the code supposed to update a timestamp if there is no timestamp provided to the function?

Expected result:
----------------
The above is properly documented and the documentation explains the context and difference between similarly looking constructs and functions.

Actual result:
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There is no documentation.