private function _closeIdleAgents()
    {
        if ($this->_gpgconf) {
            // before 2.1.13 --homedir wasn't supported, use env variable
            $env = array('GNUPGHOME' => $this->_homedir);
            $cmd = $this->_gpgconf . ' --kill gpg-agent';

            if ($process = proc_open($cmd, array(), $pipes, null, $env)) {
                proc_close($process);
            }
        }
    }