How to remove parent array keys?

Question

PHP

There is a code:

$mas = array();
        foreach ($selected as $select) {
            $mas[] = [$select['category_id'] => ['Selected' => true]];
        };
        print_r($mas);

print_r :

Array
(
    [0] => Array
        (
            [1] => Array
                (
                    [Selected] => 1
                )

        )

    [1] => Array
        (
            [2] => Array
                (
                    [Selected] => 1
                )
        )
)

How to remove 0 and 1 to make it look like this:

[1] => Array
        (
            [Selected] => 1
        )
[2] => Array
        (
            [Selected] => 1
        )

Not by hand.

Answer 1

$mas[$select['category_id']] = ['Selected' => true];
So?
Or it is impossible to form an array correctly at once according to the condition of the problem?