wp-includes/SimplePie/src/File.php:406Converts PSR-7 compatible headers into a legacy format.
$headersarraySimplePie\array<string, private function flatten_headers(array $headers): array { return array_map(function (array $values): string { return implode(',', $values); }, $headers); }Unchanged from 6.9.7 through 7.1.0.
Signature, return type and hooks compared across 3 parsed releases.
src/wp-includes/SimplePie/src/File.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.