var title = data.title;
  console.log ('title ', title);
  var custom= data['custom'];
  console.log('custom ', custom);
});

<?php
require_once "connection.php";
$url = 'https://android.googleapis.com/gcm/send';
$apiKey = 'my_api_key';
$message = array(
    'message' => 'test message',
    'title' => 'test title',
    'custom' => $myVar
);
$myVar = "34";

$registatoin_ids = array();
$selezione = "SELECT * FROM table";
$update = mysqli_query($conn, $selezione);
while ($row = $update->fetch_assoc()) {
    array_push($registatoin_ids, $row["gcm_id"]);
    echo($row["gcm_id"]);
}

$fields = array(
         'registration_ids' => $registatoin_ids,
         'data' => $message
     );


$test = json_encode($fields['data']);
echo($test);
$headers = array(   'Authorization: key=' . $apiKey,
                    'Content-Type: application/json'
);

//curl connection
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true );
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
$result = curl_exec($ch);
curl_close($ch);
echo $result;
?>