PHP apps now use Composer 2 for platform installation

Change effective on 01 February 2022

The installation of PHP, of PHP extensions and of other system packages during the build of a PHP app is now performed using Composer 2. This resolves several rare edge cases with platform package dependency resolution, notably where userland packages that declared a PHP extension as provided in their composer.json would prevent a build from succeeding.

The installation of userland dependencies from composer.lock (i.e. the regular composer install) will continue to be performed using the same Composer major version (v1 or v2) that was used to generate composer.lock.