pocketmine\utils\Config::setAll PHP Method

    public function setAll($v)
    {
        $this->config = $v;
    }

 public function saveChests()
 {
     $save = [];
     foreach ($this->chests as $chest) {
         /** @var ChestPattern $pattern */
         $pattern = $chest->getPattern();
         $save[] = ["x" => $chest->getPosition()->x, "y" => $chest->getPosition()->y, "z" => $chest->getPosition()->z, "levelName" => $chest->getPosition()->getLevel()->getName(), "patternName" => $pattern::getName(), "patternArgs" => $pattern->getPatternData()];
     }
     $this->config->setAll($save);
     $this->config->save();
 }