Change 631926 had a related patch set uploaded (by Florianschmidtwelzow; owner: Florianschmidtwelzow):
[mediawiki/core@master] Fix length parameter exceeding length of string for strcspn call

https://gerrit.wikimedia.org/r/631926

One more in Preprocessor_Hash, adding to the task description.

It seems neither the NEWS nor UPGRADING documentation for PHP 8 alpha mentions this breaking change.

As I understand it, PHP usually only introduces exceptions for scenarios that were first given a warning (using a message with a level of core warning or deprecation warning). So for them not to do this, and not to announce it, suggests it may've been done unintentionally.

I commented upstream to let ask whether this was intentional and/or whether it will be documented or backported as warning.

Anyway, the fix is fine to go ahead either way, even if PHP 8.0 might revert this, probably better not to rely on this behaviour.

Change 631926 merged by jenkins-bot:
[mediawiki/core@master] Fix PHP 8 compat with strcspn() $length parameter exceeding string

https://gerrit.wikimedia.org/r/631926

Change 631824 had a related patch set uploaded (by Reedy; owner: Florianschmidtwelzow):
[mediawiki/core@REL1_35] Fix PHP 8 compat with strcspn() $length parameter exceeding string

https://gerrit.wikimedia.org/r/631824

Change 631824 merged by jenkins-bot:
[mediawiki/core@REL1_35] Fix PHP 8 compat with strcspn() $length parameter exceeding string

https://gerrit.wikimedia.org/r/631824

I commented upstream to let ask whether this was intentional and/or whether it will be documented or backported as warning.

I agree that this looks rather unintentional. Do you've a link to the comment upstream? :)

I created a pull request to document the change at least in the UPGRADING file: https://github.com/php/php-src/pull/6381. I don't know where the actual manual lives, despite how to update it.

@Krinkle's comment is at https://github.com/php/php-src/commit/5d9ab53a5d53f11a18ae11ed31b17ff87c8d52a7#commitcomment-42942140, but I'm afraid it might get lost there.

FYI, the change in PHP8 got partially reverted. See https://github.com/php/php-src/pull/6381#issuecomment-717154087 as well as https://github.com/php/php-src/commit/d776d25a8e6763bfe8cc283a4693d7d417d50ddd.