code: protected static function _in($col, array $arr) {
          $ret = $col.' IN (';
          if (empty($arr))
              return $ret . 'null)';
          foreach ($arr as $val)
              $ret .= $val.', ';
          return substr($ret, 0, strrpos($ret, ',')) . ')';
      }
docs: 构造WHERE IN语句
  
  @param $col
  @param array $arr
  @return string