Hi Frederik

That makes sense, and shouldn't be hard at all to add.
Something like public XSLTProcessor::transformToStream(object $document, /*resource*/ $stream): int should do.
The return value is up to debate I suppose. transformToUri returns an int that gives the number of written bytes on success, and -1 on failure. While transformToXml returns false or null on error, confusingly. We aren't really a fan of int|false returns any more because of the coercion between 0 and false when doing a == comparison. So I suppose in the exceptional case where no output could be generated an exception should be thrown. Feel free to give your opinion.

Not a fan of int|falseeither. I'd prefer an exception when the method fails. So void would be sufficient. When one needs to know the number of bytes written, one could check the size of the stream before and after the operation.