How do i put items in a chest with code

If I placed a chest with this code

PHP:

$level->setBlock(new Vector3($x, $y, $z), 54);

How can I put items in that chest by code?

You need to initialize a chest tile entity for that block first.
https://github.com/PocketMine/PocketMine-MP/blob/master/src/pocketmine/block/Chest.php#L94

Chest inventory is like player inventory. But at first, you need get the chest as a tile:

PHP:

$tile = $lv->getTile(new Vector3($x,$y,$z));

Then check if this tile is really chest(not sign...):

PHP:

 if ($tile instanceof Chest){

Then, just get inventory with $tile->getInventory() and you can use addItem(Item::get($id, $meta, $cnt)), removeItem....

His chest is new. There isn't a tile entity for the cheat yet.

Edited version: "place chest, restart server and do what I wrote"

Restarting server will still not initialize the tile. You need to do that explicitly.

So, tile is automatically created only when I join game and I place the chest?

Yes. The link I posted above is the code PocketMine executed when you place a chest.

Thanks! Now, with this code I can randomly spawn chests in the whole SkyWars world. You'll no longer know where chest can appear :3

Glad to know your problem was solved. But um.. OP's question remains unsolved.

To put items in the chest, you need to acquire the inventory of the chest first.

PHP:

$chestInventory = $tileChest->getInventory();

Then set the items:

Code:

$chestInventory->setItem($itemIndex, Item::get($itemId, $itemMeta, $itemCount));

Our question is about the tile of a newly created chest, not the inventory.

I did this

PHP:

$chest = $level->createTile("chest", new Vector3($randX+4, $randY+1, $Z+4));
$chestInv = $chest->getInventory();
$config = $this->getConfig()->get("Items");
foreach($config as $i){
    $chestInv->addItem($i);
}

Would that work? (i haven't tested it yet)