 <p>Upon quick googling, <a href="http://www.phpfour.com/blog/2009/02/php-payment-gateway-library-for-paypal-authorizenet-and-2checkout/" rel="nofollow noreferrer">http://www.phpfour.com/blog/2009/02/php-payment-gateway-library-for-paypal-authorizenet-and-2checkout/</a> looks like it might work.</p>