<?php

namespace Pippin;

use ArrayAccess;

final class IPN implements ArrayAccess {

	private $data = [];

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

	private function getDataValueOrNull($key) {
		if (array_key_exists($key, $this->data)) {
			return $this->data[$key];
		}

		return null;
	}

	public function getPayerEmail() {
		return $this['payer_email'];
	}

	public function getReceiverEmail() {
		return $this['receiver_email'];
	}

	public function getTransactionId() {
		return $this['txn_id'];
	}

	// ---

	public function offsetSet($key, $value) {
		throw RuntimeException('offsetSet is unavailable for IPN, as it represents an immutable data type.');
	}

	public function offsetExists($key) {
		return isset($this->data[$key]);
	}

	public function offsetUnset($key) {
		throw RuntimeException('offsetUnset is unavailable for IPN, as it represents an immutable data type.');
	}

	public function offsetGet($key) {
		return $this->getDataValueOrNull($key);
	}

}

<?php

namespace Pippin;

use ArrayAccess;

final class IPN implements ArrayAccess {

	private $data = [];

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

	private function getDataValueOrNull($key) {
		if (array_key_exists($key, $this->data)) {
			return $this->data[$key];
		}

		return null;
	}

	public function getPayerEmail() {
		return $this->getDataValueOrNull('payer_email');
	}

	public function getReceiverEmail() {
		return $this->getDataValueOrNull('receiver_email');
	}

	public function getTransactionId() {
		return $this->getDataValueOrNull('txn_id');
	}

	// ---

	public function offsetSet($key, $value) {
		throw RuntimeException('offsetSet is unavailable for IPN, as it represents an immutable data type.');
	}

	public function offsetExists($key) {
		return isset($this->data[$key]);
	}

	public function offsetUnset($key) {
		throw RuntimeException('offsetUnset is unavailable for IPN, as it represents an immutable data type.');
	}

	public function offsetGet($key) {
		return $this->getDataValueOrNull($key);
	}

}

--- 
+++ 
@@ -21,15 +21,15 @@
 	}
 
 	public function getPayerEmail() {
-		return $this->getDataValueOrNull('payer_email');
+		return $this['payer_email'];
 	}
 
 	public function getReceiverEmail() {
-		return $this->getDataValueOrNull('receiver_email');
+		return $this['receiver_email'];
 	}
 
 	public function getTransactionId() {
-		return $this->getDataValueOrNull('txn_id');
+		return $this['txn_id'];
 	}
 
 	// ---
Use subscripts for provided methods.
