pocketmine\level\Level::addEntity PHP Method

    public function addEntity(Entity $entity)
    {
        if ($entity->getLevel() !== $this) {
            throw new LevelException("Invalid Entity level");
        }
        if ($entity instanceof Player) {
            $this->players[$entity->getId()] = $entity;
        }
        $this->entities[$entity->getId()] = $entity;
    }

 public function onActivate(Level $level, Player $player, Block $block, Block $target, $face, $fx, $fy, $fz)
 {
     if (!$target instanceof Fence) {
         return false;
     }
     foreach ($level->getChunkEntities($target->x >> 4, $target->z >> 4) as $entity) {
         if ($entity->isLeashed()) {
             if ($entity->leadHolder === $player->getId()) {
                 $nbt = new CompoundTag("", ["Pos" => new ListTag("Pos", [new DoubleTag("", $block->getX() + 0.5), new DoubleTag("", $block->getY()), new DoubleTag("", $block->getZ() + 0.5)]), "Motion" => new ListTag("Motion", [new DoubleTag("", 0), new DoubleTag("", 0), new DoubleTag("", 0)]), "Rotation" => new ListTag("Rotation", [new FloatTag("", lcg_value() * 360), new FloatTag("", 0)])]);
                 $level->addEntity($knot = new LeashKnot($level->getChunkAt($target->x >> 4, $target->z >> 4), $nbt));
                 $entity->setLeashHolder($knot);
             }
         }
     }
 }