title: Firebase unique notification key for a device group
body: <p>I'm trying to send a message to all the Android users of my app via a php website. I'm using the following code, but I have a problem. </p>
  
  <p>I don't know to get the unique key for the <code>to:</code>. I want to send it to all android users of the app. </p>
  
  <pre><code>curl -H "Content-type: application/json" -H \
      "Authorization:key=&lt;Your Api key&gt;"  -X POST \
      -d '{"to": "&lt;aUniqueKey&gt;","data": {"hello": '\
      '"This is a Firebase Cloud Messaging Device Group Message!"}}'
      https://fcm.googleapis.com/fcm/send
  </code></pre>