I've fixed the doc part, but on a closer inspection the behavior of the memory stream on past-the-end-of-file seeks could be improved.

Thanks, saw the changes in fseek.xml

What about adding streamWrapper::hasFeature(/* int */$featureId)

const STREAM_FEAT_FWDSEEK = 1;
const STREAM_FEAT_OTHER   = 2; 
//...

$stream = new File_Stream('/tmp/foo');
$stream->isSupported(STREAM_FEAT_FWDSEEK); // true
fseek($stream, 1024); // NULL bytes added

$stream = new Memory_Stream('/tmp/foo');
$stream->isSupported(STREAM_FEAT_FWDSEEK); // false
fseek($stream, 1024); // false

The goal is to have a consistent api to write to a binary stream.

I can give it some more thought, other stream 'features'.

I am making a TDataWriter class that acts as a wrapper for writing data to a seekable stream.  I was trying to figure out why it fseek was failing when moving after the end but then found this bug.  I was using memory.  It happens on temp as well even though temp is sometimes a file and sometimes not a file (aka cache below a threshold)

Writing to memory should add the null bytes as happens with files.   I have a stream wrapper, so i can easily check the stream and if fseeking is past the end, then to ftruncate the stream larger, but getting to this point is just adding difficulty where there should be none.

In some instances my class would be writing to memory and others it might be writing to a file.  Standardizing this behavior is important.

It should be documented until it is corrected.