increment doesn't apply at the first time - php

Question

i'm newbie in laravel and i'm working on a shop website and i've got stuck in the cart part of the project.
and for that i'm using a pivot table with ManyToMeny relation
the main problem is when i add to the cart for the first time it gives me always one, but in the second time it increments the third time it increments too. i searched in the pivote table structure and i found 1 as a default value.
first add
result of the first add
second add
results of the second add it increments !
i don't know how to change the value of the given 1 in the first. I don't know what to change to make the first add value like the input value.
i think the expected outcome of this code is to increment the value of 0 with the value given by input number
this is my controller
$found = false;
$user = Auth::user();
$product = produit::find($id);
if (!$product || !$user) {
abort(404);
}
if($product->rupture_stock == true){
return redirect()->back()->with('error', 'Rupture de stock');
}
$panier = $user->panier;
foreach ($panier->produits as $pr) {
if ($pr->id == $id){
$found = true;
$qt = $pr->pivot->quantite;
$first_num = $request->get('quantite');
// $first_num = $_POST['quantite'];
$second_num = 0;
$result = '';
// $qt = $_POST['quantite'];
if (is_numeric($first_num) && is_numeric($second_num)) {
$result = $first_num + $second_num;
$qt += $result;
}
$panier->produits()->updateExistingPivot($pr->id, ['quantite' => $qt]);
}
}
if (!$found) {
$panier->produits()->attach($product->id);
}
this is my blade
<form action="{{ url('add/'.$prod->id) }}" method="post" style="margin-bottom:20px" class="cart" id="product_addtocart_form" enctype='multipart/form-data'>
#csrf
<input type="number" value="" name="quantite" class="text" step="1" min="1" max="11" size="4" placeholder="" inputmode="numeric" />
<button style="border-radius: 50px; /*padding:0 20px; margin-left:40px*/" type="submit" name="add-to-cart" value="352" class="single_add_to_cart_button button alt">Ajouter</button>
</form>

Answer 1

You can do it more simple by using syncWithoutDetaching:
$user = Auth::user();
$product = produit::find($id);
if (!$product || !$user) {
abort(404);
}
if($product->rupture_stock == true){
return redirect()->back()->with('error', 'Rupture de stock');
}
$panier = $user->panier;
$panier->produits()->syncWithoutDetaching([$id => ['quantite' => (int)request('quantite')]]);
https://laravel.com/docs/8.x/eloquent-relationships#syncing-associations