@jrfnl I found the issue! 🎉

So, these failures on PHP 8.1+ happen only when the config is migrated using composer phpunit -- --migrate-configuration.

For PHPUnit tests we specify two bootstrap files:

1. bootstrap="tests/bootstrap.php" in the config file
   2../vendor/wp-cli/wp-cli-tests/tests/bootstrap.php file if it exists, passed via --bootstrap:
   https://github.com/wp-cli/wp-cli-tests/blob/bda3b5ba21db21ce9ae495795bdc471421f29641/bin/run-php-unit-tests#L13-L14

--bootstrap takes precedence over the config file. But since we also want to load the file from (1), (2) contains this code to search for such a bootstrap file and includes it if found:

https://github.com/wp-cli/wp-cli-tests/blob/bda3b5ba21db21ce9ae495795bdc471421f29641/tests/bootstrap.php#L34-L52

This regex pattern - '/bootstrap="(?P<bootstrap>.*)"/ - has a flaw in that it is greedy and matches everything until the last double quote.

The problem is, with the migrated configuration, all the attributes in the XML file are on one line, so the regex matched way more than just bootstrap="tests/bootstrap.php"

wp-cli/wp-cli-tests#224 should fix this

Much better now:

2 tests triggered 2 PHP warnings:

1) /home/runner/work/wp-cli/wp-cli/php/WP_CLI/Extractor.php:130
file(./no-such-tar.tar.gz): Failed to open stream: No such file or directory

Triggered by:

* ExtractorTest::test_err_extract_tarball
  /home/runner/work/wp-cli/wp-cli/tests/ExtractorTest.php:146

2) /home/runner/work/wp-cli/wp-cli/php/WP_CLI/Extractor.php:51
file(no-such-zip.zip): Failed to open stream: No such file or directory

Triggered by:

* ExtractorTest::test_err_extract_zip
  /home/runner/work/wp-cli/wp-cli/tests/ExtractorTest.php:216