cURL unable to get local issuer certificate - php

Question

I read some SO questions and answers but I can't solve my problem.
I've a cURL request:
function dmd_check_key($arg){
$curl = curl_init();
curl_setopt_array($curl, array(
CURLOPT_RETURNTRANSFER => 1,
CURLOPT_URL => 'https://***.de/dmd-pages-pro/dmd_key_generator.php?key='.$arg.'&website='.$_SERVER['HTTP_HOST'],
CURLOPT_USERAGENT => 'Codular Sample cURL Request',
CURLOPT_VERBOSE => 1
));
curl_setopt($curl, CURLOPT_STDERR, fopen("curl_debug.txt", "w+"));
$resp = curl_exec($curl);
curl_close($curl);
if(!get_option('dmd-pages-key-status')){add_option('dmd-pages-key-status', $resp);}else{update_option('dmd-pages-key-status', $resp);}
if(!get_option('dmd-pages-key')){add_option('dmd-pages-key', $arg);}else{update_option('dmd-pages-key', $arg);}
return $resp;
}
I got this errormessage:
Hostname was NOT found in DNS cache
Trying 00.13.133.000...
Connected to dimadirekt.de (00.13.133.000) port 443 (#0)
successfully set certificate verify locations:
CAfile: none CApath: /etc/ssl/certs
SSL certificate problem: unable to get local issuer certificate
Closing connection 0
Can someone explain me the errormessage and how I can resolve it?
I think the problem is the certificate. But on which server? I have two.
One server with the cURL file (nonssl) and one server which handles the data (ssl).
I try this settings:
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
But it is still not working.
EDIT:
With the settings line above:
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
The errormessage is gone.
Now the problem is that I don't get any response.
Now I get this response:
Hostname was NOT found in DNS cache
Trying 00.13.133.212...
Connected to ***.de (00.13.000.212) port 443 (#0)
successfully set certificate verify locations:
CAfile: none CApath: /etc/ssl/certs
SSL connection using ECDHE-RSA-AES128-GCM-SHA256
Server certificate:
subject: OU=Domain Control Validated; CN=www.***.de
start date: 2016-03-03 00:00:00 GMT
expire date: 2019-06-01 23:59:59 GMT
subjectAltName: ***.de matched
issuer: C=GB; ST=Greater Manchester; L=Salford; O=COMODO CA Limited; CN=COMODO RSA Domain Validation Secure Server CA
SSL certificate verify result: unable to get local issuer certificate (20), continuing anyway.
GET /dmd-pages-pro/dmd_key_generator.php?key=27dda19e85378bb8df73fa3f2806a30c&website=ak.dimadirekt.com
HTTP/1.1 User-Agent: Codular Sample cURL Request Host: ***.de
Accept: /
< HTTP/1.1 200 OK < Date: Tue, 31 May 2016 11:07:49 GMT
* Server Apache is not blacklisted < Server: Apache < Vary: Accept-Encoding < Transfer-Encoding: chunked < Content-Type: text/html
<
* Connection #0 to host dimadirekt.de left intact