Help Saving Credit Cards to Existing Stripe Customer IDs

Hi there. I have a core PHP project and can easily create new customers with code like this: // Create new customer $customer = \Stripe\Customer::create([ 'source' => $_POST['token'], 'email' => $user['email'], 'description' => $user['name'], // 'customer' => 'cus_xxx', ]); I also have a new Stripe Checkout form which saves the entered credit card to a new Customer ID that it creates (although I don't want it to always create new Stripe Customers, because I also want existing Customers to be able to checkout and pay). I would like to do 2 things (I can pay $50 for this): 1) Modify my new Stripe Checkout so it saves the used card to an existing customer - like if an existing Customer simply used this new Checkout to pay. Note: I tried to just set a $customer variable in the code, but it just says customer not found during the final checkout process and triggers an error - but I'm pretty sure it actually works (because I saw the card as a payment method in my Stripe Account under Customers) - so I guess I just need help NOT triggering the error so it's a successful payment (whenever I load an existing Customer ID). 2) Add something to my code that simply attaches a new card to the logged in (existing Stripe Customer ID). I would put this in my Edit Account Area. I think this is probably going to require coding something like attaching a payment method to a customer ID or something to do with payment intent or setup intent - not sure. Stripe makes it so confusing. I actually can already retrieve payment methods and display saved cards. So I'm just missing a couple of simple steps right in the middle of this whole process of trying to make it easier for customers to pay and re-use existing cards. Thanks!