Intento enviar un mensaje, pero no recibo nada. Aunque no falla, al imprimir la variable:

$result

esto vuelve "a" (no tengo idea de por qué).

El código que uso es:

private function sendMessageGcm($registration_id,$message){             
  $this->key = "xxxxxxxxxxxxxxxxxxxxxx";
  $data = array(
    "registration_id" => $registration_id,
    "data" => $message
  );
  $headers = array(
    "Content-Type:application/json",
    "Authorization:key=" . $this->key
  );
  $ch = curl_init();
  curl_setopt($ch, CURLOPT_URL, "https://android.googleapis.com/gcm/send");
  curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
  curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
  curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
  curl_setopt($ch, CURLOPT_POST, true);
  curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
  $result = curl_exec($ch);
  if($result == false) {
    echo('Curl failed: ' . curl_error($ch));
  }
  curl_close($ch);
  $rtn["code"] = "000";//means result OK
  $rtn["msg"] = "OK";
  $rtn["result"] = $result;
  return($rtn);
}