questions/41282473/ PDO not working

$sql = 'SELECT @summer := 0, wrapper.* FROM
        (SELECT
            bs_file.*,
            @summer := IFNULL(@summer,0),
            @summer := @summer+1 as summer
        FROM bs_file
    ) as wrapper
    WHERE wrapper.summer = 3';

$pdo = appDIC('run')->getDbConnection();

$stmt = $pdo->prepare($sql);
$res = $stmt->execute();
$rows = $stmt->fetchAll();

\Kint::dump($rows);


┌──────────────────────────────────────────────────────────────────────────────┐
│                                    $rows                                     │
└──────────────────────────────────────────────────────────────────────────────┘
array (1) [
    array (6) [
        '@summer := 0' => integer 0
        'id' => integer 3
        'datacol01' => string (6) "row-03"
        'whencreated' => string (19) "2016-12-22 13:16:46"
        '@summer := IFNULL(@summer,0)' => string (1) "2"
        'summer' => float 3
    ]
]