How can PHP developers ensure that the shopping cart functionality works seamlessly when transitioning between different providers or items in the cart?

When transitioning between different providers or items in the cart, PHP developers can ensure that the shopping cart functionality works seamlessly by implementing a flexible and modular architecture. This can be achieved by using interfaces and abstract classes to define common methods for interacting with different providers, as well as implementing proper error handling and logging to troubleshoot any issues that may arise during the transition.

interface PaymentProvider {
    public function processPayment($amount);
}

class PayPalProvider implements PaymentProvider {
    public function processPayment($amount) {
        // Implement PayPal payment processing logic here
    }
}

class StripeProvider implements PaymentProvider {
    public function processPayment($amount) {
        // Implement Stripe payment processing logic here
    }
}

// Example usage
$selectedProvider = new PayPalProvider();
$selectedProvider->processPayment(100);