The workaround is to comment out the lines after:

$issues = array();

/**

...

**/

in /vendor/composer/platform_check.php

or have a PHP version > 8.1 in my case (not PHP Cli), when you run update-alternatives --config php.

Don't miss to run php -v to be sure

This is a bug following a reinstallation of Composer after a bug with PHP Stan or PHP Unit (installation via composer require...). I need to check my Twig files, which are in 500, probably but not only sure, due to a package upgrade, when I changed my PHP version and ran composer update then composer update --ignore-reqs.