How can I access Stripe customer_id before a webhook triggers during Checkout Session?

When using Checkout Sessions without pre-creating a Customer, Stripe will create a new Customer during checkout. In that case, the newly created customer’s ID is only available as part of the Checkout Session (or later in the Customer object) after payment completes. Because the webhook (for example, the checkout.session.completed event) is sent immediately on completion, you won’t have a chance to look up the session on your success page first to obtain the customer ID.

The recommended solution is to pre-create (or “provision”) your Customer before creating the Checkout Session and pass the Customer ID as the session’s customer parameter. With that approach, you know the customer’s identity from the start, and the webhook events will include that same ID. For example:

PHPCopied
// Pre-create a customer
$customer = $stripe->customers->create([
  'email' => $userEmail,
  // other customer details
]);

// Create checkout session using the pre-created customer
$session = $stripe->checkout->sessions->create([
  'payment_method_types' => ['card'],
  'customer' => $customer->id,
  'line_items' => [[
    'price'    => $priceId,
    'quantity' => 1,
  ]],
  'mode' => 'subscription',
  'success_url' => $YOUR_DOMAIN . '/success?session_id={CHECKOUT_SESSION_ID}',
  'cancel_url'  => $YOUR_DOMAIN . '/cancel',
]);

If you rely on the client_reference_id parameter to pass your internal customer identifier, note that it isn’t included in the webhook payload. You can work around this limitation by either:

1. Pre-creating the Customer as shown above so that you always have the Customer ID available.
2. Storing your client reference identifier in the Checkout Session’s metadata when you create the session. Then in your webhook, retrieve the session (or use expanded data) to get that metadata.

For example:

PHPCopied
$session = $stripe->checkout->sessions->create([
  // … other parameters …
  'metadata' => [
    'internal_customer_id' => $myInternalCustomerId,
  ],
]);

Then in your webhook handler for checkout.session.completed, you could extract this metadata:

PHPCopied
$session = $event->data->object;
$internalCustomerId = $session->metadata->internal_customer_id; // if you need to correlate with your records
$customerId = $session->customer; // the newly created or existing Customer ID

In summary, to ensure you have the Customer ID available to associate with the webhook event, pre-create the Customer (or pass necessary identifiers in metadata) so that the customer information is known before the checkout completes.