protected function sign($source, $certificate, $privkey) {
        $descriptorspec = array(
            0 => array("pipe", "r"), // stdin is a pipe that the child will read from
            1 => array("pipe", "w"), // stdout is a pipe that the child will write to
            2 => array("pipe", "w")); // stderr is a file to write to

        $process = proc_open(
            'openssl smime -sign -signer ' . $certificate .
                    ' -inkey ' . $privkey .
                    ' -nochain -nocerts -outform PEM -nodetach',
            $descriptorspec, $pipes);
...
}