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[2]; //THIS LINE
break;
case "Response Reason Text":
$retVal = $valArray[3]; // THIS LINE
break;
THANKS in advance!