pocketmine\level\generator\populator\TallGrass::getHighestWorkableBlock PHP Method

    private function getHighestWorkableBlock($x, $z)
    {
        for ($y = 127; $y >= 0; --$y) {
            $b = $this->level->getBlockIdAt($x, $y, $z);
            if ($b !== Block::AIR and $b !== Block::LEAVES and $b !== Block::LEAVES2 and $b !== Block::SNOW_LAYER) {
                break;
            }
        }
        return $y === 0 ? -1 : ++$y;
    }