Как удалить родительские ключи массива?

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

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

        )

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

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