More Details:
https://support.authorize.net/s/article/MD5-Hash-End-of-Life-Signature-Key-Replacement?utm_campaign=19Q2+MD5+Hash+EOL+Merchant+Reminder&utm_medium=email&utm_source=Eloqua
I've traced the AuthNet_PHP.php pages switch parameter for case "MD5 Hash" as being $retVal = $valArray[37]; which looks to confirm the noted case.
Is there an updated file we can swap out to sort this issue?