code: public static function calculatePayloadOffset($negotiate_flags)
      {
          $offset = 0;
  
          $offset += strlen(static::SIGNATURE); // 8-byte signature
          $offset += 4; // Message-type indicator
          $offset += 4; // 32-bit Negotation flags
  
          $offset += 8; // 64-bit domain name field designator
          $offset += 8; // 64-bit client hostname field designator
  
          return $offset;
      }
docs: Calculates the offset of the "Payload" in the encoded message from the
  most-significant bit.
  
  @param int $negotiate_flags The negotiation flags encoded in the message.
  @return int The offset, in bytes.