PHP curl request with ssl/tls client certificate

cert-curl.php

<?php
//source: http://www.zedwood.com/article/php-curl-connect-with-ssl-client-certificate
class SecureRequest
{
    private m_publicCertPath; //"/public_cert.pem"
    private m_privateCertPath; //"/private.pem")
    private m_caInfoPAth; //"/etc/ssl/certs/ca-certificates.crt"
    
    public function init($publicCertPath, $privateCertPath, $caInfoPath) {
        $this->m_publicCertPath = $publicCertPAth;
        $this->m_privateCertPath = $privateCertPath;
        $this->m_caInfoPath = $caInfoPath;
    }
    
    public function post($url, $data) {
        return $this->sendRequest("POST", $url, $data);
    }
    
    public function get($url, $data) {
        $queryData = http_build_query($data);
        $urlWithData = $url . '?' . $queryData;
        return $this->sendRequest("GET", $urlWithData, array());
    }
    
    private function sendRequest($method, $url, $data) {
        //TODO: enforce https://-url
        //$data = array('key'=>'value');
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_PORT , 443);
        curl_setopt($ch, CURLOPT_VERBOSE, 0);
        curl_setopt($ch, CURLOPT_HEADER, 0);
        curl_setopt($ch, CURLOPT_SSLCERT, $this->m_publicCertPath);
        curl_setopt($ch, CURLOPT_SSLKEY, $this->m_privateCertPath);
        curl_setopt($ch, CURLOPT_CAINFO, $this->m_caInfoPath);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
                
        if( strtoupper($method) == "POST" )
        {
            curl_setopt($ch, CURLOPT_POST, 1);
            curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($data));
        }

        $response = curl_exec($ch);
        //$info =curl_errno($ch)>0 ? array("curl_error_".curl_errno($ch)=>curl_error($ch)) : curl_getinfo($ch);
        //print_r($info);
        curl_close($ch);
        return $response;
    }
}
?>

I'm having some issues with my case. the API owner sent me a public certificate "Certificate.cer" file. how do I sent it with my request as I don't have private keys and ".pem" file ?