add dimensional items in array

How can I add items in the array like this:

    array_push($arr, $end[0] => $end[1]);

I did not want to add a new array inside the existing one, but rather to add new items in the style $ key = > $ content

To stay like this:

        [A] => B
        [C] => D

And not like this:

array(
        array ( [A] => B ),
        array ( [C] => D )
)