pocketmine\level\format\leveldb\LevelDB::chunkIndex PHP Method

    public static function chunkIndex($chunkX, $chunkZ)
    {
        return Binary::writeLInt($chunkX) . Binary::writeLInt($chunkZ);
    }

 private function chunkExists($chunkX, $chunkZ)
 {
     return $this->db->get(LevelDB::chunkIndex($chunkX, $chunkZ) . self::ENTRY_VERSION) !== false;
 }