PHP 7.4 transition plan

Hi devs, For focal the server team will be transitioning PHP to 7.4 over the coming weeks.

Since version 7.0, upstream PHP has adopted a regular release cadence, with one release per year. Each release is supported for 2 years, plus a third year of security critical fixes.[1] Changes from 7.3 to 7.4 are modest, but the extra year of upstream support is a tangible benefit for the Ubuntu LTS.

While upstream released PHP 7.4 in December, Debian is still in process of landing it in Experimental. This means we'll need to temporarily go a bit ahead of Debian unstable in order to pull the stack in prior to our feature freeze. We are building a list of php packages that will need rebuilt, sync'd, and merged.

Some syntax changes and deprecated function calls have required adjustments to tests in some cases, and merging in new upstream releases in others. For FTBFS issues that will inevitably crop up, make sure to first check the Debian git repo for the package, as they've often already seen and queued up the correct fix.

1: https://www.php.net/supported-versions.php