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

    private function getHighestWorkableBlock($x, $z)
    {
        for ($y = 127; $y > 0; --$y) {
            $b = $this->level->getBlockIdAt($x, $y, $z);
            if ($b === Block::DIRT or $b === Block::GRASS or $b === Block::PODZOL) {
                break;
            } elseif ($b !== 0 and $b !== Block::SNOW_LAYER) {
                return -1;
            }
        }
        return ++$y;
    }