PHP putting an array within an array using variable

I'm just wondering, is this correct? I can't find any guide for doing it this way, but I don't know how many elements regIds will have, so it seems like it needs to be done like this. regIds is the array.

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

Edit: full script with sensitive values omitted:

<?php
$name = $_POST['name'];
$message = $_POST['message'];
$latitude = $_POST['latitude'];
$longitude = $_POST['longitude'];

$url = "dburl";
$username = "dbusername";
$password = "password";
$db = "database";
mysql_connect($url, $username, $password);
mysql_select_db($db);
$sql = mysql_query("select chatId from Players natural join Accounts where account_state = 'O' and ABS(playerLat - '$latitude') < 0.1 and ABS(playerLong - '$longitude') < 0.1");
while($row = mysql_fetch_assoc($sql))
{
    $regIds[] = $row;
}

mysql_close();

$apiKey = "key";    

// Set POST variables
$urls = 'https://android.googleapis.com/gcm/send';
$fields = array(
           'registration_ids' => $regIds,
             'data' => array( "message" => $message ),
            );
$headers = array(
          'Authorization: key=' . $apiKey,
         'Content-Type: application/json'
          );

//Open connection
$ch = curl_init();

// Set the url, number of POST vars, POST data
curl_setopt( $ch, CURLOPT_URL, $urls );
curl_setopt( $ch, CURLOPT_POST, true );
curl_setopt( $ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt( $ch, CURLOPT_RETURNTRANSFER, true );
curl_setopt( $ch, CURLOPT_POSTFIELDS, json_encode( $fields ) );

//Execute post
$result = curl_exec($ch);

// Close connection
curl_close($ch);
echo $result;
?>  

It doesn't return anything