Recurring donations with Stripe subscriptions

<?php

namespace App\Fundraising;

use App\Models\Campaign;
use Illuminate\Support\Facades\DB;
use Illuminate\Support\Facades\Http;

class MonthlyGiving
{
    /** Returns the hosted Checkout URL for a monthly gift of any amount. */
    public function start(Campaign $campaign, int $cents, string $currency): string
    {
        $session = Http::asForm()->withToken(config('services.stripe.secret'))
            ->baseUrl('https://api.stripe.com/v1')
            ->post('/checkout/sessions', [
                'mode' => 'subscription',
                'success_url' => route('campaigns.thanks', $campaign),
                'cancel_url' => route('campaigns.show', $campaign),
                'line_items[0][quantity]' => 1,
                'line_items[0][price_data][currency]' => strtolower($currency),
                'line_items[0][price_data][unit_amount]' => $cents,
                'line_items[0][price_data][recurring][interval]' => 'month',
                'line_items[0][price_data][product_data][name]' => "Monthly gift to {$campaign->title}",
                // Copied onto the subscription and every invoice it produces.
                'subscription_data[metadata][campaign_id]' => $campaign->id,
            ])->throw()->json();

        return $session['url'];
    }

    /** Webhook branch for `invoice.paid`: first payment and every renewal. */
    public function onInvoicePaid(array $invoice): void
    {
        $campaignId = $invoice['subscription_details']['metadata']['campaign_id'] ?? null;

        if (! $campaignId || ($invoice['amount_paid'] ?? 0) <= 0) {
            return;
        }

        // Unique invoice id: a redelivered event inserts nothing.
        DB::table('donations')->insertOrIgnore([
            'campaign_id' => $campaignId,
            'amount_cents' => $invoice['amount_paid'],
            'currency' => $invoice['currency'],
            'provider_invoice_id' => $invoice['id'],
            'recurring' => true,
            'created_at' => now(),
        ]);
    }
}