wp-includes/rss.php:280$str1$str2optional"" function concat (&$str1, $str2="") { if (!isset($str1) ) { $str1=""; } $str1 .= $str2; }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/rss.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.