Also a diffusion callsign or whatever else might be needed; it's vaguely mentioned on https://www.mediawiki.org/wiki/Gerrit/New_repositories but the appropriate project for the request isn't indicated.
For more info, see https://gerrit.wikimedia.org/r/#/c/246255/ and https://www.mediawiki.org/wiki/Php-session-serializer.
Requested project php-session-serializer has been created: https://phabricator.wikimedia.org/project/profile/1581/
Please encourage interested people to visit the project and to join the project as members, and to subscribe themselves to the project in order to receive updates. Enjoy!
Also a diffusion callsign or whatever else might be needed
No idea about that part, I'd expect that to be handled by T616 instead.