Stripe - PHP - Update Subscription

gistfile1.php

// Retrieve the customer:
$customer = Stripe_Customer::retrieve($customer_id);

// Retrieve the subscription being updated:
$subscription = $customer->subscriptions->retrieve($subscription_id);

// Change the subscription settings:
$subscription->trial_end = strtotime('first day of next month 12:00');

// Save the changes:
$subscription->save();