pocketmine\level\generator\biome\Biome::register PHP Method

    protected static function register($id, Biome $biome)
    {
        self::$biomes[(int) $id] = $biome;
        $biome->setId((int) $id);
        $biome->grassColor = self::generateBiomeColor($biome->getTemperature(), $biome->getRainfall());
        $flowerPopFound = false;
        foreach ($biome->getPopulators() as $populator) {
            if ($populator instanceof Flower) {
                $flowerPopFound = true;
                break;
            }
        }
        if ($flowerPopFound === false) {
            $flower = new Flower();
            $biome->addPopulator($flower);
        }
    }