Do not use multibyte capable functions to calculate length of headers

Sometimes mb_strlen() will recognize a certain sequence of bits as a
multibyte character and act according to what is was designed to do:
avoid this behaviour by using a function that does not have support for
multibyte.

src/Client.php

@@ -108,6 +108,7 @@ public function sendSocketData(WritableContentInterface $object): string
         $writableString = $object->createSendableMessage();
         $sizeOfString = \strlen($writableString);
         $writtenBytes = fwrite($this->socket, $writableString, $sizeOfString);
+        // $this->logger->debug('Sent string', ['binaryString' => str2bin($writableString)]); // Handy for debugging
         if ($writtenBytes !== $sizeOfString) {
             $this->logger->error('Written bytes do NOT correspond with size of string!', [
                 'writtenBytes' => $writtenBytes,

src/Internals/WritableContent.php

@@ -99,7 +99,7 @@ final public function createSendableMessage(): string
         $this->logger->debug('Creating variable header', ['variableHeader' => base64_encode($variableHeader)]);
         $payload = $this->createPayload();
         $this->logger->debug('Creating payload', ['payload' => base64_encode($payload)]);
-        $fixedHeader = $this->createFixedHeader(mb_strlen($variableHeader . $payload));
+        $fixedHeader = $this->createFixedHeader(\strlen($variableHeader . $payload));
         $this->logger->debug('Created fixed header', ['fixedHeader' => base64_encode($fixedHeader)]);
 
         return $fixedHeader . $variableHeader . $payload;

src/Protocol/Subscribe.php

@@ -84,6 +84,7 @@ public function setPacketIdentifier(int $packetIdentifier): Subscribe
         }
 
         $this->packetIdentifier = $packetIdentifier;
+        $this->logger->debug('Setting packet identifier', ['current' => $this->packetIdentifier]);
 
         return $this;
     }