PHP Abstract Classes & Interfaces: Architectural Contracts & Polymorphism

When building large web applications or team projects, senior software architects need a way to enforce standards across different developers. If five developers are creating different payment gateways (PayPal, Stripe, Razorpay), how do you guarantee that all five gateways provide the exact same method names? PHP provides two powerful architectural tools: Abstract Classes and Interfaces.

Side-by-Side Comparison: Abstract Class vs Interface

1. Abstract Class Implementation

<?php
    // Abstract Base Class
    abstract class PaymentGateway {
        protected float $feePercentage = 2.5;

        // Concrete method with working implementation (inherited by all)
        public function calculateTransactionFee(float $amount): float {
            return ($amount * $this->feePercentage) / 100;
        }

        // Abstract method: Child MUST implement custom charge logic
        abstract public function processCharge(float $amount): bool;
    }

    class StripeGateway extends PaymentGateway {
        public function processCharge(float $amount): bool {
            echo "Charging ₹$amount securely through Stripe API.<br>";
            return true;
        }
    }

    $stripe = new StripeGateway();
    $stripe->processCharge(1500);
    echo "Fee: ₹" . $stripe->calculateTransactionFee(1500);
?>

2. Interface Implementation & Polymorphism

<?php
    interface NotifierInterface {
        public function sendAlert(string $recipient, string $message): bool;
    }

    interface LoggableInterface {
        public function writeLog(string $entry): void;
    }

    // Implementing MULTIPLE interfaces
    class SmsNotificationService implements NotifierInterface, LoggableInterface {
        public function sendAlert(string $recipient, string $message): bool {
            echo "SMS sent to $recipient: $message<br>";
            return true;
        }

        public function writeLog(string $entry): void {
            file_put_contents("notifications.log", $entry . "\n", FILE_APPEND);
        }
    }

    // Polymorphism in action: Type-hinting against the interface contract!
    function notifyCustomer(NotifierInterface $notifier, string $phone, string $msg) {
        $notifier->sendAlert($phone, $msg);
    }

    notifyCustomer(new SmsNotificationService(), "+919876543210", "Your BCA exam hall ticket is ready!");
?>

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