In case it helps anyone else here’s the problem i had, and (what seems to be) a solution.

I was posting to http://www.sandbox.paypal.com/cgi-bin/webscr and getting the expected reply (tx etc)
I was then creating a curl post back to that address, but paypal wasn’t talking back to me.
I used the curl_getinfo($ch, CURLINFO_HTTP_CODE) from your article and found a 302 HTTP error.
I changed the url in my curl post from http to https and all is now well, so far..

Thanks again!!

The simulator tool worked totally fine when I was testing, and it was quite simple to use. Are you sure the URL you give it is correct? Have you tried to visit the URL in your browser to see if it exists and that it doesn’t throw any errors or anything like that? Checked the domain through for example http://www.downforeveryoneorjustme.com to see if it is accessible from other places than your machine? What do you do with the debug statements and log write lines? Remember you need to store them in a file or something and not just echo them out. And if you do store them in a file, have you made sure this actually works?

I have no idea what’s wrong with your code, but there are some thoughts at least.. Good luck

I understand the IPN concept, and the conversation between client, server and PayPal, and the difference between http and https. My problem is getting *anything* at all from the simulator. My handler url is accessible from the rest of the world and i have debug statements and log write lines all over the code, but asking PayPal to simulate an IPN call always results in the same error message “IPN delivery failed. Unable to connect to the specified URL. Please verify the URL and try again.” despite sometimes hitting my handler page and sometimes not.

The simulator tool seems to be as crappy as PayPal’s documentation.

What probably could be wrong? I am stuck here for quite some time now and any help would be much appreciated.