ParagonIE\Halite\Stream\MutableFile::writeBytes PHP Method

    public function writeBytes(string $buf, int $num = null) : int
    {
        $bufSize = CryptoUtil::safeStrlen($buf);
        if ($num === null || $num > $bufSize) {
            $num = $bufSize;
        }
        if ($num < 0) {
            throw new CryptoException\CannotPerformOperation('num < 0');
        }
        $remaining = $num;
        do {
            if ($remaining <= 0) {
                break;
            }
            $written = \fwrite($this->fp, $buf, $remaining);
            if ($written === false) {
                throw new CryptoException\FileAccessDenied('Could not write to the file');
            }
            $buf = CryptoUtil::safeSubstr($buf, $written, null);
            $this->pos += $written;
            $this->stat = \fstat($this->fp);
            $remaining -= $written;
        } while ($remaining > 0);
        return $num;
    }

 /**
  * Stream decryption - Do not call directly
  * 
  * @param ReadOnlyFile $input
  * @param MutableFile $output
  * @param Key $encKey
  * @param string $nonce
  * @param resource $mac (hash context)
  * @param Config $config
  * @throws FileAlert\AccessDenied
  */
 private static final function streamDecrypt(ReadOnlyFile $input, MutableFile $output, KeyInterface $encKey, $nonce, $mac, Config $config, array &$chunk_macs)
 {
     if (!$encKey instanceof EncryptionKey) {
         throw new \ParagonIE\Halite\Alerts\InvalidKey('Argument 3: Expected an instance of EncryptionKey');
     }
     $start = $input->getPos();
     $cipher_end = $input->getSize() - $config->MAC_SIZE;
     // Begin the streaming decryption
     $input->reset($start);
     while ($input->remainingBytes() > $config->MAC_SIZE) {
         if ($input->getPos() + $config->BUFFER > $cipher_end) {
             $read = $input->readBytes($cipher_end - $input->getPos());
         } else {
             $read = $input->readBytes($config->BUFFER);
         }
         \hash_update($mac, $read);
         $calcMAC = \hash_copy($mac);
         if ($calcMAC === false) {
             throw new CryptoException\CannotPerformOperation('An unknown error has occurred');
         }
         $calc = \hash_final($calcMAC, true);
         if (empty($chunk_macs)) {
             throw new CryptoException\InvalidMessage('Invalid message authentication code');
         } else {
             $chkmac = \array_shift($chunk_macs);
             if (!\hash_equals($chkmac, $calc)) {
                 throw new CryptoException\InvalidMessage('Invalid message authentication code');
             }
         }
         $decrypted = \Sodium\crypto_stream_xor($read, $nonce, $encKey->get());
         $output->writeBytes($decrypted);
         \Sodium\increment($nonce);
     }
     \Sodium\memzero($nonce);
     return true;
 }