Handling Variable Amount (Donation) With Stripe & PHP

I need to allow users to enter custom amounts using Stripe. They enter what they want to pay in a input. Then I have a script that collects the amount below the default stripe checkout button. But how am I supposed to handle the charge server-side on charge.php?

donation.php

    //user enter custom amount
    <input type="number" class='form-control' 
    id="custom-donation-amount" 
    placeholder="$50.00" min="0" step="10.00 " />

    //default stripe checkout button
      <script
        src="https://checkout.stripe.com/checkout.js" class="stripe-button"
        data-key="pk_test_XXXXXXXXXXXXXXX"
        data-amount="0"
        data-name="ORGANIZATION"
        data-description="Saving Penguins"
        data-image="logo.png"
        data-locale="auto"
        data-zip-code="true"
        data-billing-address="true"
        data-label="DONATE"
        data-currency="usd">
      </script>

//script to collect variable amount
  <script type="text/javascript">
    $(function() {
        $('.donate-button').click(function(event) {
            var amount = $('#custom-donation-amount').val();        
            $('.stripe-button').attr('data-amount', amount)
        });
    });
</script>

charge.php

<?php require_once('./config.php');

  $token  = $_POST['stripeToken'];
  $email  = $_POST['stripeEmail'];

  $customer = \Stripe\Customer::create(array(
      'source'  => $token,
      'email' => $email)
  );

  $charge = \Stripe\Charge::create(array(
      'customer' => $customer->id,
      'amount'   => $amount,
      'currency' => 'usd'
  ));

   header("Location: confirmation.php");
die();


?>