code: public static function set(string $fileName, array $content)
      {
          try {
              $json = Yaml::dump($content, 2);
              if (file_put_contents($fileName, $json) === false) {
                  throw new FileNotWritable('can\'t write to "' . $fileName . '"');
              }
          } catch (DumpException $e) {
              throw new BadUse('Config::set() content parameter can\'t be dump to YAML');
          }
          self::$files[$fileName] = $content;
      }
docs: Write a file content. Save it internally.
  
  @static
  @throws FileNotWritable if file can't be written
  @param  string          $fileName file name
  @param  array           $content  file content