title: validating certificate chains for PayPal IPN
body: <p>I am trying to integrate <a href="https://github.com/Quixotix/PHP-PayPal-IPN" rel="nofollow">PHP-PayPal-IPN</a> (great library btw, you can see the blog post on usage <a href="http://www.micahcarrick.com/paypal-ipn-with-php.html" rel="nofollow">here</a>). </p>
  
  <p>Since security is essential here, I have checked the source code. I found no apparent problems with it, but I would also like to check the PayPal certificate chain that is bundled with the library: <a href="https://github.com/Quixotix/PHP-PayPal-IPN/tree/master/cert" rel="nofollow">https://github.com/Quixotix/PHP-PayPal-IPN/tree/master/cert</a></p>
  
  <p>How can I check that the certificate chain is correct? (<em>correct</em> == "it allows only connections from PayPal and from nowhere else")</p>