pocketmine\utils\Binary::writeLDouble PHP Method

    public static function writeLDouble($value)
    {
        return ENDIANNESS === self::BIG_ENDIAN ? strrev(pack("d", $value)) : pack("d", $value);
    }

 public function putDouble($v)
 {
     $this->buffer .= $this->endianness === self::BIG_ENDIAN ? Binary::writeDouble($v) : Binary::writeLDouble($v);
 }