PHP 8.0 transition plan

[Bryce Harrington]

Hi devs,

For impish the server team will be transitioning PHP to 8.0 over the
coming weeks. (LP: #1927264) [0]

Since version 7.0, upstream PHP has adopted a regular release cadence[1],
with one release per year. Each release is supported for 2 years, plus a
third year of security critical fixes. PHP 8.0 was available last cycle
but since changes from 7.4 to 8.0 were significant[2], we opted to postpone
it to the 21.10 cycle and focused instead on resolving some phpunit
8.5->9.5 transition troubles. Landing PHP 8.0 this cycle will enable it
to receive testing in a release prior to 22.04 LTS.

php8.0 has now been sync'd into impish from Debian experimental and set
as the default PHP.  This breaks the PHP stack[3] and starts the actual
transition.

Most or all of the PHP stack will need rebuilt at this point. This is
phased due to package dependencies - for the php7.4 transition we
started with php-propro, php-apcu, and php-msgpack; followed by
php-apcu-bc, php-imagick, and php-igbinary; and then the rest of the
stack. I anticipate php8.0's transition will proceed similarly.

Typically during PHP transitions, packages will fail to build or fail
autopkgtests. Deprecated functionality[4] has been a common case and
likely will be so for php8.0 as well[5]. There may also be
phpunit-related issues in packages that didn't get re-built or re-tested
since phpunit 9.5 transitioned.

Thanks,
Bryce

0: https://wiki.ubuntu.com/ServerTeam/Transition/Php8.0
1: https://www.php.net/supported-versions.php
2: https://stitcher.io/blog/new-in-php-8
3: https://people.canonical.com/~ubuntu-archive/proposed-migration/update_excuses.html#php-defaults
4: https://wiki.php.net/rfc/deprecations_php_7_4
5: https://stitcher.io/blog/new-in-php-8