Receiving 'data' part of push notifications

Hi there,
I am trying to receive, with my standalone AppGyver app, the ‘data’ part of notifications.

What I send from my backend is:

    $url = '';
    $api_key = 'AAAAI....Mp2s';

$fields = array (
        'to' => 'dPTX0FCaSmW....uViMIFSidED',
        'notification' => array(
            'body' => 'This is the body',
            'title' => 'Notification's title'
        'data' => array(
            'key01' => 'value01',
            'key02' => 'value02'

    $headers = array(

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
    $result = curl_exec($ch);

And what I receive is something like:

{"android":{},"body":"This is the body","title":"Notification's title"}

Does anyone know how I can retrieve the ‘dats’ part of the notification?

Many thanks