PHP Interfaces

Intermediate~30 min read

Interfaces define contracts that classes must follow. Unlike abstract classes, a class can implement multiple interfaces - perfect for defining capabilities!

<?php
// Interfaces

// Interface - contract that classes must follow
interface PaymentInterface
{
    public function processPayment($amount);
    public function refund($transactionId);
}

class CreditCardPayment implements PaymentInterface
{
    public function processPayment($amount)
    {
        echo "Processing credit card payment: $$amount\n";
        return true;
    }

public function refund($transactionId)
    {
        echo "Refunding transaction: $transactionId\n";
        return true;
    }
}

class PayPalPayment implements PaymentInterface
{
    public function processPayment($amount)
    {
        echo "Processing PayPal payment: $$amount\n";
        return true;
    }

public function refund($transactionId)
    {
        echo "Refunding PayPal transaction: $transactionId\n";
        return true;
    }
}

// Multiple interfaces
interface Loggable
{
    public function log($message);
}

class AdvancedPayment implements PaymentInterface, Loggable
{
    public function processPayment($amount)
    {
        $this->log("Payment processed: $$amount");
        return true;
    }

public function refund($transactionId)
    {
        $this->log("Refund processed: $transactionId");
        return true;
    }

public function log($message)
    {
        echo "[LOG] $message\n";
    }
}

$cc = new CreditCardPayment();
$cc->processPayment(100);

$advanced = new AdvancedPayment();
$advanced->processPayment(200);

Output

Click Run to execute your code

PHP Interfaces - Contract and Implementation

Defining an Interface

<?php
interface PaymentInterface {
    public function processPayment($amount);
    public function refund($transactionId);
}
?>

Implementing an Interface

<?php
class CreditCardPayment implements PaymentInterface {
    public function processPayment($amount) {
        echo "Processing $$amount";
    }
    
    public function refund($transactionId) {
        echo "Refunding $transactionId";
    }
}
?>

Multiple Interfaces

<?php
class Payment implements PaymentInterface, Loggable {
    // Must implement all methods from both interfaces
}
?>

PHP Access Modifiers - public, protected, private

Three Access Levels

interface: Contract definition
implements: Follow the contract
Multiple: Can implement many interfaces
All public: Interface methods are always public

What's Next?

Next, learn about Traits - horizontal code reuse without inheritance!

Previous Abstract Classes

Next Traits

Defining an Interface

Implementing an Interface

Multiple Interfaces

Three Access Levels

What's Next?

Enjoying these tutorials?