I am using Google Cloud Messaging from last one year with PHP as server side script and it is working fine on all app until I hasn't sent notification to one of the may application for one month then after that while sending notification to same app I gets blank response like
so I assumed that google may have deactivated my API key so I regenerated my API key from developer console and replaced regenerated key with old key on server side and at that time it had worked I was able to send message again but after two week again I am getting blank response like given above and this time I was sent notification to same app everyday.
From this related SO question. It suggested to make sure that it's retrieving the POST variable in your PHP code.
Try to replace this:
$message = array("Notice" => $_POST['message']);
$message = array("Notice" => "testing");
You can also check on this tutorial and thread on how to integrate GCM in PHP backend properly.
Problem is solved after importing project into firebase. Server API Key is auto generated by google services and after using those keys on server site i am able to send notification.