Curl authentication error on https server using CURLOPT_SSLCERT - php

Question

I appreciate this has been asked a number of times but after looking through the answers I'm still getting the following error:
Curl error: Peer certificate cannot be authenticated with known CA certificates
I have the below code, which works on a non-https version of the site (different server), however, it won't work on the live site running https. My assumption is that it's trying to verify against the SSL certificate on the site, rather than the .pem file specified in $cert_path.
I'll be honest, I'm a bit confused between SSLCERT and the CAINFO however I have added the following to my php.ini file and checked to make sure they're set correctly, which they are.
// php.ini
curl.cainfo = "/path/to/.pem";
openssl.cafile = "/path/to/.pem";
As I said before, the following works on the stage version of the site, just not the live one. Any help would be greatly appreciated.
// php curl snippet
$cert_path = '/path/to/.pem';
$options = array(
CURLOPT_SSL_VERIFYHOST => true,
CURLOPT_SSL_VERIFYPEER => true,
CURLOPT_SSLCERT => $cert_path,
CURLOPT_SSLCERTPASSWD => $password,
CURLOPT_CAINFO => $cert_path,
CURLOPT_HTTPAUTH => CURLAUTH_ANY,
CURLOPT_URL => $url,
CURLOPT_RETURNTRANSFER => true,
CURLOPT_POST => true,
CURLOPT_POSTFIELDS => $xml_post_string,
CURLOPT_HTTPHEADER => $headers
);
curl_setopt_array( $ch, $options );
If I set VERIFYPEER to false it does connect but my assumption is that this wouldn't be passing the data securely?
Thank you.

Answer 1

I've managed to get this working however there are a few things to note which may help someone in the future.
The site was hosted with 123reg and I was informed by their support that third-party SSL certificates are not accepted. Although I was told this ultimately my code below did work.
I regenerated my .pem file so this too could have worked in my favour.
My final code as as follows: I also stripped out the php.ini code entirely.
// These two variables will differ depending on your payload for Curl
$xml_post_string = '...';
$headers = array();
// This will differ based on your endpoint
$url = 'endpoint.url';
$password = 'password';
$cert_path = '/path/to/.pem';
$ch = curl_init();
$options = array(
CURLOPT_SSL_VERIFYHOST => 2,
CURLOPT_SSL_VERIFYPEER => true,
CURLOPT_SSLCERT => $cert_path,
CURLOPT_SSLCERTTYPE => "PEM",
CURLOPT_SSLCERTPASSWD => $password,
CURLOPT_HTTPAUTH => CURLAUTH_ANY,
CURLOPT_URL => $url,
CURLOPT_RETURNTRANSFER => true,
CURLOPT_POST => true,
CURLOPT_POSTFIELDS => $xml_post_string,
CURLOPT_HTTPHEADER => $headers
);
curl_setopt_array( $ch, $options );
$response = curl_exec($ch);
curl_close($ch);