code: protected function renderSeparator($spanLeft, $spanMid, $spanMidMid, $spanRight)
      {
          $separator = $spanLeft;
          foreach ($this->_columnWidths as $index => $rowSize) {
              if ($index !== 0) {
                  $separator .= $spanMid;
              }
              $separator .= str_repeat($spanMidMid, $rowSize);
          }
          $separator .= $spanRight . "\n";
          return $separator;
      }
docs: Renders separator.
  
  @param string $spanLeft character for left border
  @param string $spanMid character for middle border
  @param string $spanMidMid character for middle-middle border
  @param string $spanRight character for right border
  @return string the generated separator row
  @see \yii\console\widgets\Table::render()