fix strings to be binary

Author: aldas

examples/example_parallel_requests_amp.php

@@ -62,7 +62,7 @@ function requestWithAmp(array $requests)
             try {
                 yield $socket->write($request);
 
-                $data = '';
+                $data = b'';
                 while (null !== $chunk = yield $socket->read()) {
                     // there are rare cases when MODBUS packet is received by multiple fragmented TCP packets and it could
                     // take PHP multiple reads from stream to get full packet. So we concatenate data and check if all that

examples/example_parallel_requests_reactphp.php

@@ -51,7 +51,7 @@ function requestWithReactPhp(array $requests)
 
         $connector->connect($request->getUri())->then(
             function (React\Socket\ConnectionInterface $connection) use ($request, $promise) {
-                $receivedData = '';
+                $receivedData = b'';
 
                 $connection->write($request);
 

src/Network/StreamHandler.php

@@ -68,7 +68,7 @@ protected function receiveFrom(array $readStreams, float $timeout = null, $logge
                         if ($logger) {
                             $logger->debug("Stream {$streamId} @ index: {$streamIndex} received data: ", unpack('H*', $data));
                         }
-                        $packetData = ($result[$streamIndex] ?? '') . $data;
+                        $packetData = ($result[$streamIndex] ?? b'') . $data;
                         $result[$streamIndex] = $packetData;
 
                         // MODBUS SPECIFIC PART: if we received complete packet to at least one stream we were waiting