As per https://gerrit.wikimedia.org/r/c/mediawiki/vendor/+/667051/1/composer/platform_check.php as part of rebuilding vendor on composer 2.0 (T266421: Rebuild mediawiki-vendor on composer 2.0), which is run from autoload_real.php::getLoader() - https://gerrit.wikimedia.org/r/c/mediawiki/vendor/+/667051/1/composer/autoload_real.php

We should make sure the MW platform checks (PHP version etc) are run before the composer ones.

I think for index.php (at least) it's fine...

index.php does PHPVersionCheck, then WebStart, which does Setup, which eventually loads vendor/autoload.php which then loads vendor/composer/autoload_real.php, which will run composer/platform_check.php....