CURL Webhooks unable to verify: How do I go about debugging this?

[oak2000]

I'm using the code CURL example:
https://developer.paypal.com/docs/classic/ipn/ht-ipn/

If I use the tool IPN Simulator (web accept)
https://developer.paypal.com/developer/webhooksSimulator/
I get

$res == "VERIFIED"

in Sandbox and Production
I get 

$res == "INVALID"

If I do

file_put_contents('testpost.txt', file_get_contents('php://input'));

I do see a POST of the payment confirmation in son formate

even with

curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);

i get

$res == "INVALID"

How do I go about debugging this?