wp-includes/pomo/streams.php:93$input_stringstring$startint$lengthintstring public function substr( $input_string, $start, $length ) { if ( $this->is_overloaded ) { return mb_substr( $input_string, $start, $length, 'ascii' ); } else { return substr( $input_string, $start, $length ); } }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/pomo/streams.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.