“未定义的变量”，当尝试回显通过mysql_fetch_array获取的数组时

下面是我的代码：

session_start();
include_once "config.inc.php";
$tbl_name="members";
 $username=$_SESSION['username'];
$sql = "SELECT quiz1mark, quiz2mark, quiz3mark FROM $tbl_name WHERE username='$username'";
$query = mysql_query($sql);
$data = mysql_fetch_array($query, MYSQL_ASSOC);

然后，在页面下方：

<p><?php echo $data; ?></p>

这将生成一个通知：Notice: Undefined variable: data in F:'xampp'htdocs'quiz_home.php on line 35

不过，我已经清楚地定义了变量，并且不确定导致此问题的原因。

***Contents of config.inc.php***:
$host="localhost";
$username="root";
$password="";   //the default installation of xampp does not include a mysql password
$db_name="bda";
//connect to the database using the above information (variables)
mysql_connect("$host", "$username", "$password")or die("cannot connect to server");
mysql_select_db("$db_name")or die("cannot select database");