title: PHP Firebase Clood Messaging (FCM) error missing registration
body: <p>I am building a push notification from my server to FCM server. I getting following errors: <code>Error=MissingRegistration</code></p>
  
  <p>My PHP code is given below.</p>
  
  <pre><code>function send($id,$title,$text){
    $msg = [
      'title' =&gt; $title,
      'body' =&gt; $text,
      'icon' =&gt; 'myicon',
       'sound' =&gt; 'mySound'
    ];
  
    $fields = [
      'to' =&gt; $id,
      'notification' =&gt; $msg
    ];
  
    $headers = [
      'Authorization: key=' . $api_key,
      'Content-Type: application/json'
    ];
  
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt ( $ch, CURLOPT_POST, true );
    curl_setopt ( $ch, CURLOPT_HTTPHEADER, $headers );
    curl_setopt ( $ch, CURLOPT_RETURNTRANSFER, true );
    curl_setopt ( $ch, CURLOPT_POSTFIELDS, $fields );
    $result = curl_exec ( $ch );
    curl_close ( $ch );
    echo $result;
  }
  </code></pre>
  
  <p>I am calling function like this:</p>
  
  <pre><code>send($id,$title,$text);
  </code></pre>