Why stream_get_contents not working with ssh2?

Question

I use next code for receiving time of server Debian:

<?php
$connection = ssh2_connect('123.123.123.123', 23452);
ssh2_auth_password($connection, 'kvn', 'kvn');
$stream = ssh2_exec($connection, '/bin/date');
$err_stream = ssh2_fetch_stream($stream, SSH2_STREAM_STDERR);
$dio_stream = ssh2_fetch_stream($stream, SSH2_STREAM_STDIO);
stream_set_blocking($err_stream, true);
stream_set_blocking($dio_stream, true);
$result_err = stream_get_contents($err_stream);
$result_dio = stream_get_contents($dio_stream);

echo ($result_dio);
?>

But code crashes on string:

$result_err = stream_get_contents($err_stream)

I tried fgets() but code crashes too. Why it this possible? How I can receive string from stream? Thank you!