Following up on this again. The site owner didn't tell me that this still hasn't been working for months. Is there any other possible cause? I have verified that their authorize.net account is in good standing and their codes are accurate. The error references this code:
case "Response Reason Code":
$retVal = $valArray; //THIS LINE
case "Response Reason Text":
$retVal = $valArray; // THIS LINE
THANKS in advance!