PHP Replace multidimensional array keys

I would like to replace some keys, my array is:

Array
(
    [0] => Array
        (
            [0] => test1
            [1] => test2
            [2] => test3
            [3] => test4
            [4] => test5
            [5] => test6
        )

    [1] => Array
        (
            [0] => test7
            [1] => test8
            [2] => test9
            [3] => test10
            [4] => test11
            [5] => test12
        )

)

My wish:

Array
(
    [45] => Array
        (
            [0] => test1
            [1] => test2
            [2] => test3
            [3] => test4
            [4] => test5
            [5] => test6
        )

    [51] => Array
        (
            [0] => test7
            [1] => test8
            [2] => test9
            [3] => test10
            [4] => test11
            [5] => test12
        )

)

45 and 51 are examples.

How can I do that? I tried array_flip() then array_search() but PHP says: "Can only flip STRING and INTEGER values!"

Thank's anyway. Regards

---

$old_array = array(
    0 => array(0=>'test1',1=>'test2',2=>'test3',3=>'test4',4=>'test5',5=>'test6'),
    1 => array(0=>'test1',1=>'test2',2=>'test3',3=>'test4',4=>'test5',5=>'test6')
);
$new_array = array(
    45=>$old_array[0],
    51=>$old_array[1]
);
var_dump($new_array);