pocketmine\block\Block::isTopFacingSurfaceSolid PHP Method

    public function isTopFacingSurfaceSolid()
    {
        if ($this->isSolid()) {
            return true;
        } else {
            if ($this instanceof Stair and ($this->getDamage() & 4) == 4) {
                return true;
            } elseif ($this instanceof Slab and ($this->getDamage() & 8) == 8) {
                return true;
            } elseif ($this instanceof SnowLayer and ($this->getDamage() & 7) == 7) {
                return true;
            }
        }
        return false;
    }