strategy.php

<?php

interface PaymentProvider
{
    public function purchase();
}

class BrainTreePaymentProvider implements PaymentProvider
{
    public function purchase()
    {
        var_dump('Purchasing through Brain tree');
    }
}

class SagePayPaymentProvider implements PaymentProvider
{
    public function purchase()
    {
        var_dump('Purchasing through Sagepay');
    }
}

class Booking
{
    public function create(PaymentProvider $paymentProvider)
    {
        $paymentProvider->purchase();
    }
}

$booking = new Booking();
$booking->create(new BrainTreePaymentProvider);
$booking->create(new SagePayPaymentProvider);