When the DeprecationErrorHandler checks if it should colorize the output, in certain circumstances it can trigger the following warning:

 Warning: stream_isatty(): cannot cast a filtered stream on this system in /app/vendor/symfony/phpunit-bridge/DeprecationErrorHandler.php on line 411

This is triggered by some admittedly poor behavior inside of php.stream_isatty checks by doing a cast and looking for a failure and one of those failures logs a warning. You generally don't want to trigger warnings during a capabilities check(that's why I'm filing this) but PHP is doing just that.

This can lead to test suite failures since warnings are generally treated as failures by most test suites.

I think a test case to trigger the deprecation handler might be tricky but a simplified test case to trigger the warning in php looks like this.

class strtoupper_filterextends php_user_filter {  #[\ReturnTypeWillChange]functionfilter($in,$out, &$consumed,$closing)  {while ($bucket =stream_bucket_make_writeable($in)) {$bucket->data =strtoupper($bucket->data);$consumed +=$bucket->datalen;stream_bucket_append($out,$bucket);    }returnPSFS_PASS_ON;  }}$stdout =fopen('php://stdout','wb');stream_filter_register('capture', strtoupper_filter::class);stream_filter_append($stdout,'capture');var_export(stream_isatty($stdout));

https://3v4l.org/0EHT4

A real world example of a test suite that is wrapping stdout with a stream wrapper during testing is Drupal.Feature addition

References:
stream_isatty definition
cast line that triggers warning