$stripe = new \Stripe\StripeClient('sk_test_51KX5RA...CV00KMzNeBbs
sk_test_51KX5RAHpz0yDkAJjgP65eUwiAJMY1mHeA4LoeX6162wxO6RDyGFZaQR6mOY1X9e7PpVAXkC1AT1HWNfE7ri3HRCV00KMzNeBbs
');
$stripe->customers->update(
  'cus_NffrFeUfNV2Hib',
  ['metadata' => ['order_id' => '6735']]
);