pocketmine\level\format\generic\BaseFullChunk::setBiomeId PHP Method

    public function setBiomeId($x, $z, $biomeId)
    {
        $this->hasChanged = true;
        $this->biomeColors[($z << 4) + $x] = $this->biomeColors[($z << 4) + $x] & 0xffffff | $biomeId << 24;
    }