php中的函数arraymultisort更改了我的数组的键

我有这个数组：

$array["4E-952778"][0]['fileName'] = "File 1";
$array["4E-952778"][0]['product'] = "Muse On Demand";
$array["4E-952778"][1]['fileName'] = "File 2";
$array["4E-952778"][1]['product'] = "Muse On Demand";   
$array["15210"][0]['fileName'] = "File 3";
$array["15210"][0]['product'] = "4Manager"; 
$array["15210"][1]['fileName'] = "File 4";
$array["15210"][1]['product'] = "4Manager";
$products = array();
foreach ($array as $key => $row) {
    $products[$key] = $row[0]['product'];       
}
array_multisort($products, SORT_ASC, $array);
print_r($array);

结果是：

Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [fileName] => File 3
                    [product] => 4Manager
                )
            [1] => Array
                (
                    [fileName] => File 4
                    [product] => 4Manager
                )
        )
    [4E-952778] => Array
        (
            [0] => Array
                (
                    [fileName] => File 1
                    [product] => Muse On Demand
                )
            [1] => Array
                (
                    [fileName] => File 2
                    [product] => Muse On Demand
                )
        )
)

正如你所观察到的，函数array_multisort()改变了键：15210到0为什么会改变？