PHPMailer::sign PHP Method

    public function sign($cert_filename, $key_filename, $key_pass, $extracerts_filename = '')
    {
        $this->sign_cert_file = $cert_filename;
        $this->sign_key_file = $key_filename;
        $this->sign_key_pass = $key_pass;
        $this->sign_extracerts_file = $extracerts_filename;
    }

 /**
  * S/MIME Signing tests.
  */
 public function testSigning()
 {
     $this->Mail->Subject .= ': S/MIME signing';
     $this->Mail->Body = 'This message is S/MIME signed.';
     $this->buildBody();
     $dn = ['countryName' => 'UK', 'stateOrProvinceName' => 'Here', 'localityName' => 'There', 'organizationName' => 'PHP', 'organizationalUnitName' => 'PHPMailer', 'commonName' => 'PHPMailer Test', 'emailAddress' => '*****@*****.**'];
     $password = '******';
     $certfile = 'certfile.txt';
     $keyfile = 'keyfile.txt';
     //Make a new key pair
     $pk = openssl_pkey_new();
     //Create a certificate signing request
     $csr = openssl_csr_new($dn, $pk);
     //Create a self-signed cert
     $cert = openssl_csr_sign($csr, null, $pk, 1);
     //Save the cert
     openssl_x509_export($cert, $certout);
     file_put_contents($certfile, $certout);
     //Save the key
     openssl_pkey_export($pk, $pkeyout, $password);
     file_put_contents($keyfile, $pkeyout);
     $this->Mail->sign($certfile, $keyfile, $password);
     $this->assertTrue($this->Mail->send(), 'S/MIME signing failed');
     unlink($certfile);
     unlink($keyfile);
 }