A basic Processwire module to send payments to PayPal and process the IPN response using the website payments standard API.

PaymentGatewayPayPal.module

<?php

class PaymentGatewayPayPal extends WireData implements Module, ConfigurableModule {

	/*
	 * Module details, defaults and initialisation
	 *
	 */

	public static function getModuleInfo()
	{
		return array(
			'title' => 'PayPal Payment Processing',
			'version' => 001,
			'summary' => 'Send payments to PayPal and validate their IPN responses',
			'singular' => true,
			'autoload' => true
		);
	}

	static public function getDefaultData() {
		return array(
			'paypalAccount' => null,
			'paypalDeveloperAccount' => null,
			'paypalUrl' => "https://www.paypal.com/cgi-bin/webscr?",
			'sandbox' => false,
			'ipnListener' => false,
			'completedPage' => false,
			'currency' => null
		);
	}
	
	public function __construct() {
	   foreach(self::getDefaultData() as $key => $value) {
		   $this->$key = $value;
	   }
	}
	
	public function init() {
		
		// Set localised title
		$this->title = $this->_("PayPal");
		
		// Set PayPal URL depending on whether we are in sandbox mode
		$paypalHost = $this->sandbox ? "www.sandbox.paypal.com" : "www.paypal.com";
		$this->paypalUrl = "https://$paypalHost/cgi-bin/webscr?";
		
		// Set ipnListener URL
		$this->ipnListener = $this->pages->get("/")->httpUrl . "ipnlistener/";
		
		// Intercept requests to 'ipnlistener' and process the IPN
		if (strpos($_SERVER['REQUEST_URI'], wire('config')->urls->root . 'ipnlistener') !== FALSE) {
			$this->processIpn($_POST);
			exit;
		}
	}

	/*
	 * Send a payment to PayPal using Website Payments Standard. Full documentation can be found at:
	 * https://www.x.com/developers/paypal/documentation-tools/paypal-payments-standard/integration-guide/Appx_websitestandard_htmlvariables
	 *
	 * $payment = array(
	 * 	'quantity' => 1,
	 * 	'item_name'	=> "Socks",
	 * 	'amount' => "2.50",
	 * 	'invoice'	=>	"123",
	 * 	'cancel_return' => $page->httpUrl . "cancelled/",
	 * 	'currency_code' => "GBP"
	 * );
	 * $modules->PaymentsPayPal->beginPayment($payment);
	 *
	 *
	 */
	 
	public function beginPayment($payment) {

		// Defaults
		
		$account = $this->sandbox ? $this->paypalDeveloperAccount : $this->paypalAccount;		
		$return = $this->completedPage ? $this->pages->get($this->completedPage)->httpUrl : $this->pages->get("/")->httpUrl;
		
		// Create a payment with the defaults and merge it with the provided payment
				
		$paypalObj = array(
			'cmd'		=>	"_xclick",
			'business' 	=> 	$account,
			'return' => $return,
			'notify_url' => $this->ipnListener
		);
		
		$paypalObj = array_merge($paypalObj,$payment);
		
		// Send the payment to PayPal
		
		$endpoint = $this->paypalUrl . http_build_query($paypalObj);
		$this->session->redirect($endpoint,false);

	}
	
	
	/*
	 *	Process the IPN
	 *
	 */
	
	public function ___processIpn($post) {
	
		if(!$this->validateIpn($post)) {
			
			mail($this->config->adminEmail, "Invalid IPN response", print_r($post, true));
			return false;
		
		}
		
		// Testing
		mail($this->config->adminEmail, "Valid IPN response", print_r($post, true));
		
		return true;
	
	}
	
		
	/*
	 *	Validate the IPN by returning it to PayPal
	 *
	 */
	
	public function validateIpn($post) {
	
		$data = array(
			'cmd'	=> "_notify-validate"
		);

		$url = $this->paypalUrl;
		$data = array_merge($post,$data);
		
		$options = array(
			'http' => array(
				'method'  => 'POST',
				'content' => http_build_query($data)
			)
		);
		
		$context  = stream_context_create($options);
		$result = file_get_contents($url, false, $context);
		
		if (strcmp ($result, "VERIFIED") == 0) {
			return true;
		}
		
		return false;
		
	}
	
	/*
	 * Configuration
	 *
	 */
	 
	static public function getModuleConfigInputfields(Array $data) {
	
		// Get the defaults and alias $modules
		
		$data = array_merge(self::getDefaultData(), $data);
		$modules = wire('modules');
		
		// Build config form
		
		$fields = new InputfieldWrapper();
		
		$f = $modules->get("InputfieldEmail");
		$f->name = 'paypalAccount';
		$f->value = $data['paypalAccount'];
		$f->label = "PayPal Account Email";
		$f->description = "The email address of the account you would like to pay money into.";
		$f->required = 1;
		$fields->add($f);
		
		$f = $modules->get("InputfieldEmail");
		$f->name = 'paypalDeveloperAccount';
		$f->value = $data['paypalDeveloperAccount'];
		$f->label = "PayPal Developer Account Email";
		$f->description = "A developer account for use with the PayPal developer sandbox.";
		$fields->add($f);
		
		$f = $modules->get("InputfieldCheckbox");
		$f->name = 'sandbox';
		$f->label = "Use PayPal developer sandbox?";
		$f->value = 1;
		$f->attr('checked', empty($data['sandbox']) ? '' : 'checked');
		$fields->add($f);
		
		$f = $modules->get("InputfieldPageListSelect");
		$f->attr('name', 'completedUrl');
		$f->attr('value', $data['completedUrl']);
		$f->derefAsPage = 1; // single page reference
		$f->label = "Thank you page";
		$f->description = 'The page the user is returned to after completion of the payment.';
		$fields->add($f);
		
		$f = $modules->get("InputfieldText");
		$f->name = 'currency';
		$f->value = $data['currency'];
		$f->label = "Currency code";
		$f->description = "Three letter currency code. The PayPal default is USD.";
		$fields->add($f);
		
		return $fields;
		
	}
	
}