CashierSubscriptions.php

<?php

class CashierSubscriptions implements Subscriptions
{
  public function startSubscription($billable, $subscription, $plan)
  {
    return $billable->newSubscription($subscription, $plan)->create();
  }
}

FakeSubscriptions.php

<?php

use Laravel\Cashier\Subscription;

class FakeSubscriptions implements Subscriptions
{
  public function startSubscription($billable, $subscription, $plan)
  {
    return Subscription::create([
      'name' => $subscription,
      'stripe_id' => '',
      'stripe_status' => 'active',
      'stripe_plan' => $plan,
    ]);
  }
}

Subscriptions.php

<?php

interface Subscriptions
{
  public function startSubscription($billable, $subscription, $plan);
}