Maybe one of your SSL Certificate is somehow not verify well by Curl. One of my API domain get strange error this morning.

--2016-11-30 06:34:52--  https://xxxx.api/
Resolving xxxx.api (xxxx.api)... 13.23.10.58
Connecting to xxxx.api (xxxx.api)|13.23.10.58|:443... connected.
ERROR: The certificate of `xxxx.api' is not trusted.
ERROR: The certificate of `xxxx.api' hasn't got a known issuer.

And I don’t know exactly, maybe lib-curl is broken, php-curl is broken or something else broken. But passing some parameter to CURL to ignore those buggy invalid verification is good enough to solve this silly problem.

According to documentation: to verify host or peer certificate you need to specify alternate certificates with the CURLOPT_CAINFO option or a certificate directory can be specified with the CURLOPT_CAPATH option.

Also look at CURLOPT_SSL_VERIFYHOST: 1 to check the existence of a common name in the SSL peer certificate. 2 to check the existence of a common name and also verify that it matches the hostname provided.

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