You shouldn't need the development kit for this. Not many people modify the payment scripts which is why there's not much experience of this on the forum.
To add another hidden input I would try adding the extra input to the hidden values text. i.e.
Try Changing this:
$sHiddenValues .= "<INPUT TYPE=HIDDEN NAME=\"MERCHANTID\" VALUE=\"$sMerchantID\">\n";
to
$sHiddenValues .= "<INPUT TYPE=HIDDEN NAME=\"MERCHANTID\" VALUE=\"$sMerchantID\">\n<input type="hidden" name="accId1" value="MERCHANTCODE">\n";
Mike
Edit: OK. Looking at this again you might need to escape the " character with a \ i.e.
$sHiddenValues .= "<INPUT TYPE=HIDDEN NAME=\"MERCHANTID\" VALUE=\"$sMerchantID\">\n<input type=\"hidden\" name=\"accId1\" value=\"MERCHANTCODE\">\n";
To add another hidden input I would try adding the extra input to the hidden values text. i.e.
Try Changing this:
$sHiddenValues .= "<INPUT TYPE=HIDDEN NAME=\"MERCHANTID\" VALUE=\"$sMerchantID\">\n";
to
$sHiddenValues .= "<INPUT TYPE=HIDDEN NAME=\"MERCHANTID\" VALUE=\"$sMerchantID\">\n<input type="hidden" name="accId1" value="MERCHANTCODE">\n";
Mike
Edit: OK. Looking at this again you might need to escape the " character with a \ i.e.
$sHiddenValues .= "<INPUT TYPE=HIDDEN NAME=\"MERCHANTID\" VALUE=\"$sMerchantID\">\n<input type=\"hidden\" name=\"accId1\" value=\"MERCHANTCODE\">\n";
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