Q:

Wordpress addon unable to fetch RSS feed from Joomla website

I am developing a new website with Wordpress (4.8.1). I've installed the WP RSS Aggregator addon (4.11.2) to get RSS feed from a previously existing website on Joomla (2.5.17).
The Wordpress installation is on a GNU/Linux Debian Stretch (9.1) OS, served in https by an Apache (2.4.25) server. Joomla is on Squeeze (6.0.10) and Apache (2.2.16) ; the website is served in https but it seems it doesn't work on the RSS feeds (the URL is in https, but the browser tells the connection is not secured).
Here is a snippet of the Apache configuration :
<VirtualHost *:80>
        ServerName intranet.cdg44.fr
        ServerAlias i2.cdg44.fr
        Redirect permanent / https://intranet.cdg44.fr/
</VirtualHost>

<VirtualHost *:443>
        ServerName intranet.cdg44.fr
        SSLEngine On
        SSLCertificateFile /etc/ssl/certs/cdg44.pem
        SSLCertificateKeyFile /etc/ssl/private/ca.key
        <Directory />
                Options FollowSymLinks
                AllowOverride None
                AuthType Kerberos
                AuthName "Kerberos Login"
                KrbMethodNegotiate On
                KrbMethodK5Passwd On
                KrbAuthRealms CDG44.FR
                Krb5KeyTab /etc/krb5.keytab
                require valid-user
        </Directory>
</VirtualHost>

(For those two websites, I use authentication with Negociate).
Th WP RSS Aggregator addon tells me :
Failed to fetch the RSS feed. Error: cURL error 60: SSL certificate problem: unable to get local issuer certificate

What can I do to solve my issue ?
EDIT :
Trying to get programmatically the RSS feed :
$ch = curl_init();
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
file_get_contents("https://address.website.com/index.php?option=com_content&view=category&id=27&Itemid=241&format=feed&type=rss");

Gives this error (by xdebug) :
Warning: file_get_contents(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed in /home/pyledevehat/workspace/intranet/wp-content/themes/themename/functions.php on line 197

My php.ini is well configured :
allow_url_fopen = On

A:

I solved my problem with this code :
$url = "https://address.website.com/index.php?option=com_content&view=category&id=27&Itemid=241&format=feed&type=rss";
$ch = curl_init();
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HEADER, false);
$content = curl_exec($ch);
curl_close($ch);

