Additional filesystem functions that have the same or similar problems:

Optional $context arg emits warning when explicit NULL is passed to all of: copy(), mkdir(), rename(), rmdir(), unlink()

fgetss(), fgets() - NULL passed to $length is cast to int(0), emits warning
file_get_contents() - NULL passed to $maxlen results in empty string returned

> fwrite() should accept null for chunk size

That would constitute a BC break, though.

> for an example where this is annoying.

  $written = @\fwrite($stream, $data, $chunkSize ?: $length);

How would that be a BC break?

> How would that be a BC break?

Presently, passing NULL as third parameter to fwrite() causes
nothing to be written.  If I understand you correctly, you want
that to change so that the whole $string is written instead.

bad idea - the param is defined as int which means in weak mode it will be casted and NULL becomes 0 (what you don't want) while with strict_types it is a fatal error to pass NULL - that is the expected behavior from each and every function and should stay consistent