wp-includes/PHPMailer/PHPMailer.php:4854Quoted-Printable-encode a DKIM header.
$txtstringstring public function DKIM_QP($txt) { $line = ''; $len = strlen($txt); for ($i = 0; $i < $len; ++$i) { $ord = ord($txt[$i]); if (((0x21 <= $ord) && ($ord <= 0x3A)) || $ord === 0x3C || ((0x3E <= $ord) && ($ord <= 0x7E))) { $line .= $txt[$i]; } else { $line .= '=' . sprintf('%02X', $ord); } } return $line; }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/PHPMailer.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.