Validate:
$totalRows_checkRS
not:
$row_checkRS['UserID']
your current validation would only pass if the UserID of the user was -1 or 0, which will never happen and it always says it isn't valid... like you mention.
If you validate the number of rows instead of the row result I think it will work:
$WAFV_Errors .= WAValidateNM($totalRows_checkRS . "",-1,0,"",",.",true,1);