To authenticate web clients using a browser is no difficulty when one has imported the PKI certificate in the browser.
But how can we do that with a php driven process and use it as client. Here is some short PHP code that will do exactly that.

Note: the CA, PKI certificate(CRT) and private key(KEY) files must be in PEM format for this to work.

    $ch = curl_init('');
    curl_setopt($ch, CURLOPT_PORT , 443);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, true);
    curl_setopt($ch, CURLOPT_SSLCERT, "/etc/ssl/certs/example_CRT.pem");
    curl_setopt($ch, CURLOPT_SSLKEY, "/etc/ssl/certs/example_KEY.pem");
    curl_setopt($ch, CURLOPT_CAINFO, "/etc/ssl/certs/example_CA.pem");
    $output = curl_exec($ch);
    $response = curl_getinfo($ch);
    var_dump ($response);
    var_dump ($output);