Open/Closed Principle (OCP).

OpenClosedPrincipal.php

<?php

// Bad demo
$paypal = new Paypal();

$paymentManager = new PaymentManager($paypal);

$paymentManager->process();

class PaymentManager
{
    protected $paypal;

    public function __construct(Paypal $paypal)
    {
        $this->paypal = $paypal;
    }

    public function process()
    {
        $this->paypal->processPayment();

        // ...and other payment stuff
    }
}

class Paypal
{
    public function processPayment()
    {
        // ...process the payment with PayPal
    }
}

// Good demo
$paypal = new Paypal();
$creditCard = new CreditCard();

$paymentManager = new PaymentManager($paypal);

$paymentManager->process();

class PaymentManager
{
    protected $paymentMethod;

    public function __construct(PaymentMethodInterface $paymentMethod)
    {
        $this->paymentMethod = $paymentMethod;
    }

    public function process()
    {
        $this->paymentMethod->processPayment();

        // ...and other payment stuff
    }
}

interface PaymentMethodInterface
{
    public function processPayment();
}

class Paypal implements PaymentMethodInterface
{
    public function processPayment()
    {
        // ...process the payment with PayPal
    }
}

class CreditCard implements PaymentMethodInterface
{
    public function processPayment()
    {
        // ...process the payment with PayPal
    }
}