None, I'm not arguing for the solution. Proxying through a PHP script just to keep using Google Analytics is overkill when private self-hosted solutions exist. I'm simply showing how you can anonymize IPs even from yourself, if the goal is to anonymize from only Google and not the server it could be useful across sessions.

The solution being overkill does not mean my first comment 'That's why you use a salt, which is what I assume is meant by "non-reversibly"' is wrong.