Payment method declined but user is still subscribed

Question

I have a website built on the PHP Laravel Framework. I use Stripe to process recurring subscription fees for the service via Laravel Cashier. This is the code for the SubscriptionController.php:

class SubscriptionController extends Controller
{
    function __construct()
    {
        $this->middleware('auth');
    }

    public function payment()
    {
        $availablePlans =[
           'monthly-4_99' => "Monthly",
           'yearly' => "Yearly"
        ];

        $data = [
            'intent' => auth()->user()->createSetupIntent(),
            'plans'=> $availablePlans
        ];

        return view('user.subscribe')->with($data);
    }

    public function subscribe(Request $request)
    {
        $user = auth()->user();
        $paymentMethod = $request->payment_method;
        $planId = $request->plan;
        $coupon = $request->coupon;

        if ($coupon == 'Free60') {
            $user->newSubscription('plan', $planId)->trialDays(60)->create($paymentMethod);
        } else if ($coupon == '1YrEA') {
            $planId = 'monthly-4_99';
            $user->newSubscription('plan', 'monthly-4_99')->trialDays(365)->create($paymentMethod);
        } else if ($coupon == 'LAT-44') {
            $planId = 'monthly-4_99';
            $user->newSubscription('plan', 'monthly-4_99')->withCoupon($coupon)->create($paymentMethod);
        } else {
            $user->newSubscription('plan', $planId)->create($paymentMethod);
        }

        if ($planId == 'monthly-4_99') {
            return response([
                'success_url'=> redirect()->intended('memorials')->with('success', 'You are now subscribed to a monthly plan.')->getTargetUrl()
            ]);
        } else if ($planId == 'yearly') {
            $user = auth()->user();
            User::find($user->id)->update(['free_qrplaque' => 'yes']);

            return response([
                'success_url'=> redirect()->intended('memorials')->with('success', 'You are now subscribed to a ' . $planId . ' plan.')->getTargetUrl()
            ]);
        }
    }
}

This works as it should, but there is one problem I just discovered. When a user's payment information is no longer valid, the user remains subscribed and has access to all premium features of the service. When I try to cancel the subscription by clicking the "Unsubscribe" button, the website gives me a 500 Server Error. Here is the code for canceling a subscription:

public function cancel()
    {
        $user = auth()->user();

        $user->subscription('plan')->cancel();

        return redirect()->back()->with("success", "Your subscription has been canceled.");
    }

When the user logs in and views the dashboard, I have some code that checks if they are subscribed. If they are not, I process the changes to reflect a free user's capabilities (not being able to publish memorials). Here is the code:

public function index()
{
    $user = auth()->user();

    if (!$user->subscribed('plan') && $user->memorials->count() > 0) {
        foreach ($user->memorials as $memorial) {
            $memorial->update(['status' => 'unpublished']);
        }
    }

    return view('memorials');
}

I'm assuming this code is not targeting the correct variable to check if the user is still subscribed, or if I need to add additional code for when a user's payment method is declined to make the necessary changes in the database to reflect being unsubscribed.

If anyone can help me fix this issue, it would be greatly appreciated.