code: public function getStream(string $mode = 'r+')
      {
          if (!in_array($mode, ['r+','w','w+','a+','x','x+','c+',true])) {
              throw new RuntimeException(sprintf('unable to open stream for memory-storage in non-write mode "%s"', $mode), 500);
          }
  
          $stream = \fopen('php://memory', $mode);
  
          if ($stream === false) {
              throw new RuntimeException('failed to open stream', 500);
          }
  
          // pre-fill stream
          if ($this->content !== null) {
              \fwrite($stream, $this->content);
              \rewind($stream);
          }
          return $stream;
      }
docs: this stream is <b>READONLY!</b>
  @inheritDoc
  @throws RuntimeException