public static function prep($upc) { $upc = parent::prep($upc); if ($upc[0] !== '0') { if (strlen($upc) <= 7) { $upc = '0' . $upc; } else { throw new RangeException('UPC-E must belong to the 0th number system (Start with 0).', 2304); } // More than max of 09999999 } if (strlen($upc) === 7) { $upc = $upc . self::check_sum($upc); } return $upc; }