code: private function renderCell($columnNumber, $value, $filler, $style = '')
      {
          $output = [];
          $width = $this->getWidth($columnNumber);
          $output[] = $filler;
          while ($this->length($value) < $width) {
              $value .= $filler;
          }
          $output[] = Style::applyStyle($value, $style);
          $output[] = $filler;
          return implode('', $output);
      }
docs: Render cell.
  
  @param integer $columnNumber
  @param string $value
  @param string $filler
  @param string $style Default ''.
  @return string