code: public function appendSetter($key, $value = null, $comment = null, $export = false)
      {
          $line = $this->formatter->formatSetterLine($key, $value, $comment, $export);
  
          return $this->appendLine($line);
      }
docs: Append one setter to buffer
  
  @param  string       $key
  @param  string|null  $value
  @param  string|null  $comment
  @param  boolean      $export
  
  @return DotenvWriter