Field不是JSON数组

Question

我收到通知：“registration_ids”字段不是收到的JSON数组消息。任何人都可以帮我解决这个问题吗？

如果我手动注册代码，那么它可以完美地运行

function sendNotification($id_user, $msg_type) {

    define('API_ACCESS_KEY','AIzaSyDLJmlsR5Px8t4BoGqs1UsGPY1R8TmUIxI');

    $userData = mysql_query('SELECT * FROM tbl_users WHERE id = "'.$id_user.'"') or die(mysql_error());

    if(mysql_num_rows($userData) == 1) {
        $userData = mysql_fetch_array($userData);
        $message = $title = '';

        switch($msg_type) {
            case "new_order":

                $title = "New order added.";
                $message = "New order has been added by ".$userData["username"];

            break;

            default:
                $message = $title = 'Default Message & Title';
            break;
        }

        $msg = array
        (
            'message'   => $message,
            'title'     => $title,
            'subtitle'  => '',
            'tickerText'    => '',
            'vibrate'   => 1,
            'sound'     => 1,
            'largeIcon' => 'large_icon',
            'smallIcon' => 'small_icon'
        );

        $parentQuery = mysql_query('SELECT id_admin FROM tbl_user_relation WHERE id_user = "'.$id_user.'"') or die(mysql_error());

        if(mysql_num_rows($parentQuery) == 1) {
            $parentData = mysql_fetch_array($parentQuery);  

            $regQ = mysql_query('SELECT registration_id FROM tbl_users WHERE id = "'.$parentData['id_admin'].'" ') or die(mysql_error());

            if(mysql_num_rows($regQ) == 1) {

                $parentData = mysql_fetch_array($regQ);

                $fields = array
                (
                    'registration_ids'  => $parentData,
                    'data'          => $msg
                );

                $headers = array
                (
                    'Authorization: key=' . API_ACCESS_KEY,
                    'Content-Type: application/json'
                );

                $ch = curl_init();
                curl_setopt( $ch, CURLOPT_URL , 'https://android.googleapis.com/gcm/send' );
                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;

            }
        }
    }    
}