php supprime les données dans la table de données

define ('DB_HOST', 'localhost');
define ('DB_USER', 'root');
define ('DB_PASS', 'root');
define ('DB_NAME', 'php');
define ('DB_CHAR', 'utf8');
$db = @mysqli_connect(DB_HOST, DB_USER, DB_PASS);
if (mysqli_connect_errno($db)) {
    exit('连接失败'.mysqli_connect_error($db));
}
mysqli_select_db($db, DB_NAME);
mysqli_set_charset($db, DB_CHAR);

$sql = "DELETE FROM staff  WHERE staff_id=7";

if (mysqli_query($db, $sql)) {
    if (mysqli_affected_rows($db) > 0) {
        //返回受影响的记录数
        echo '成功的删除了'.mysqli_affected_rows($db).'条记录';
    } else {
        echo '没有记录被删除';
    }
} else {
    exit(mysqli_errno($db).':'.mysqli_error($db));
}

mysqli_close($db);