An introduction taken from Taylor's Laravel book and extended to display the useful nature of interfaces.

interfaces.php

<?php
header("Content-Type: text/plain");

interface BillerInterface {
  public function bill(array $user, $amount);
}

interface BillingNotifierInterface {
	public function notify(array $user, $amount);
}

class NotifyByEmail implements BillingNotifierInterface {
	public function notify(array $user, $amount) {
		echo "Send email for ".$user["name"]." of $amount".PHP_EOL;
	}
}

class NotifyByMessage implements BillingNotifierInterface {
	public function notify(array $user, $amount) {
		echo "Display message for ".$user["name"]." of $amount".PHP_EOL;
	}
}

class NotifyBySMS implements BillingNotifierInterface {
	public function notify(array $user, $amount) {
		echo "Send SMS for ".$user["name"]." of $amount".PHP_EOL;
	}
}

class StripeBiller implements BillerInterface {
	public function __construct(BillingNotifierInterface $notifier)
	{
		$this->notifier = $notifier;
	}

	public function bill(array $user, $amount)
	{
		// Bill the user via Stripe...
		echo "Bill via Stripe".PHP_EOL;
		$this->notifier->notify($user, $amount);
	}
}

class PaypalBiller implements BillerInterface {
	public function __construct(BillingNotifierInterface $notifier)
	{
		$this->notifier = $notifier;
	}

	public function bill(array $user, $amount)
	{
		echo "Bill via Paypal".PHP_EOL;
		$this->notifier->notify($user, $amount);
	}
}

class EPDQBiller implements BillerInterface {
	public function __construct(BillingNotifierInterface $notifier)
	{
		$this->notifier = $notifier;
	}

	public function bill(array $user, $amount)
	{
		echo "Bill via ePDQ".PHP_EOL;
		$this->notifier->notify($user, $amount);
	}
}

$newBill = new StripeBiller(new NotifyBySMS);
$newBill->bill(
	array(
		"name" => "Scott Wilcox"
	),
	"123.89"
);

$newBill = new PaypalBiller(new NotifyByMessage);
$newBill->bill(
	array(
		"name" => "Scott Wilcox"
	),
	"123.89"
);

$newBill = new EPDQBiller(new NotifyByEmail);
$newBill->bill(
	array(
		"name" => "Scott Wilcox"
	),
	"123.89"
);