It was added in 2016 as part of SessionManager (Ic1ffea74f3, later
re-attributed to Ibb3e023e4e) by Anomie. It wasn't used at the time.
Basically, it is used for cases where we expect no logs, and the
presence of logs will then produce a failing test. The point is to do
something that PHPUnit will "complain" about by showing it. Emitting
a notice or warning is one way, another is to simply echo it, since
output is also unexpected and shown. The somewhat arbitrary mapping
from log levels to PHP error codes isn't used.
Later in 2017, he introduced the same class in php-session-serializer
(I2ebf59414ef9a9) where this part was already removed and replaced
by "echo". The version in core still used trigger_error.
== Why now ==
Triggering E_USER_ERROR is deprecated in PHP 8.4+.
Bug: T379445
Change-Id: Id1e1db80bc8fea39cd192716597e5e4a6f4966b0
(cherry picked from commit 5a1a8c14bd418c846498cd4ed99b77e45a0fcca1)
50b0aadc6a