public function DKIM_Sign($s) {
    $privKeyStr = file_get_contents($this->DKIM_private);
    if ($this->DKIM_passphrase!='') {
      $privKey = openssl_pkey_get_private($privKeyStr,$this->DKIM_passphrase);
    } else {
      $privKey = $privKeyStr;
    }
    if (openssl_sign($s, $signature, $privKey)) {
      return base64_encode($signature);
    }
  }