gistfile1.php

<?php

GoCardless::$environment = 'sandbox'; // Whilst you're testing...

$account_details = array(
  'app_id'        => 'app id',
  'app_secret'    => 'app secret',
  'merchant_id'   => 'merchant id',
  'access_token'  => 'access token'
);

GoCardless::set_account_details($account_details);

$bill_details = array(
  'amount'           => $_POST['amount'], // You might want to do some filtering on this first to check it's the right format
  'name'             => "Donation to My Charity"
);

// Generate the url
$bill_url = GoCardless::new_bill_url($bill_details);

// Redirect to the URL that was generated
die(header("Location: " . $bill_url));

?>