Convert multidimensional array into single array

$array = Array
    (
        [0] => Array
            (
                [plan] => basic
            )

        [1] => Array
            (
                [plan] => small
            )

        [2] => Array
            (
                [plan] => novice
            )

        [3] => Array
            (
                [plan] => professional
            )

        [4] => Array
            (
                [plan] => master
            )

        [5] => Array
            (
                [plan] => promo
            )

        [6] => Array
            (
                [plan] => newplan
            )

    );

$array = array_map('current', $array);


Array (
[0] => basic
[1] => small
[2] => novice
[3] => professional
[4] => master
[5] => promo
[6] => newplan
)