Yuvi said that sticky sessions are not currently implemented, but can be using Redis as the session handler. It should be something like this in the INI:
session.save_handler = redis session.save_path = "tcp://tools-redis:6379?prefix=tool_name_random_number"
Please reopen with more information on what to do if it's still relevant. Like "add documentation for php users", or similar.
Here's the current redis documentation: https://wikitech.wikimedia.org/wiki/Help:Toolforge/Redis_for_Toolforge
Note that it is unsafe to use the shared redis for user sessions in most cases (https://wikitech.wikimedia.org/wiki/Help:Toolforge/Redis_for_Toolforge#Security).
The Cloud-Services project tag is not intended to have any tasks. Please check the list on https://phabricator.wikimedia.org/project/profile/832/ and replace it with a more specific project tag to this task. Thanks!