( resource channel, int streamid )
Fetches an alternate substream associated with an SSH2 channel stream
identified by streamid. The SSH2 protocol
currently defines only one substream, STDERR, which has a substream ID
of SSH2_STREAM_STDERR (defined as 1).
Example 1. Opening a shell and retrieving the stderr stream associated with it.
$connection = ssh2_connect('shell.example.com', 22);
ssh2_auth_password($connection, 'username', 'password');
$stdio_stream = ssh2_shell($connection);
$stderr_stream = ssh2_fetch_stream($stdio_stream, SSH2_STREAM_STDERR);