PHP SSH Warning: stream_get_contents(): Failure 'would block' (-9)" - php

Question

I got a Warning from my code
Warning: stream_get_contents(): Failure 'would block' (-9) in /var/www/html/public_html/site.php on line 51
It is thrown only at this command:
service apache2 reload
My code is:
public function exec( $cmd, &$output = '' ) {
$connection = ssh2_connect( '*********', 22 );
ssh2_auth_password( $connection, '********', '******+*' );
$stream = ssh2_exec( $connection, $cmd );
$errorStream = ssh2_fetch_stream( $stream, SSH2_STREAM_STDERR );
stream_set_blocking( $errorStream, TRUE );
stream_set_blocking( $stream, TRUE );
var_dump( $cmd );
echo '<br>';
$output = stream_get_contents( $stream );
$error = stream_get_contents( $errorStream );
if ( strlen( $error ) > 0 ) {
var_dump( $error );
}
fclose( $errorStream );
fclose( $stream );
}
The code is starting at line 39, the warning is on the line with the
$output = stream_get_contents( $stream );
Any solution against the warning ?
EDIT
The function is in a class that is a part of a webinterface to manage the vhosts
Some Informations about the Server:
Apache
root#development:/# apache2 -v
Server version: Apache/2.4.10 (Debian)
Server built: Aug 28 2015 16:28:08
PHP
root#development:/# php -v
PHP 5.6.13-0+deb8u1 (cli) (built: Sep 7 2015 13:38:37)
Copyright (c) 1997-2015 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2015 Zend Technologies
with Zend OPcache v7.0.6-dev, Copyright (c) 1999-2015, by Zend Technologies
OS
root#development:/# uname -a
Linux development 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt11-1+deb8u3 (2015-08-04) x86_64 GNU/Linux
root#development:/# lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 8.2 (jessie)
Release: 8.2
Codename: jessie