You can confir the issue checking the curl error
$error = curl_error($curl)
SHould say somathing like: SSL certificate problem: unable to get local issuer certificate
you can download the file here
To test it, configure the curl to use those certificates. Add this configuration
curl_setopt($curl, CURLOPT_CAINFO, 'your/absolute/favorite/path/cacert.pem');
With it, the ssl issue should e fixed.
If you want, you can set a default value for CAINFO. TO do that, open your php.ini
file,
find the configuration curl.cainfo
and uncomment and set the absolute path of you .pem file.
A good place to this file is inside the php folder, together to php.ini