exec($this->buildCurlCommand($url, $data, $headers));

protected function buildCurlCommand($url, $data, $headers)
{
    // TODO(dcramer): support ca_cert
    $cmd = $this->curl_path.' -X POST ';
    foreach ($headers as $key => $value) {
        $cmd .= '-H ' . escapeshellarg($key.': '.$value). ' ';
    }
    $cmd .= '-d ' . escapeshellarg($data) . ' ';
    $cmd .= escapeshellarg($url) . ' ';
    $cmd .= '-m 5 ';  // 5 second timeout for the whole process (connect + send)
    if (!$this->verify_ssl) {
        $cmd .= '-k ';
    }
    $cmd .= '> /dev/null 2>&1 &'; // ensure exec returns immediately while curl runs in the background
    return $cmd;
}