php array sort numeric first and alphabetic last

I have an array:

Array
(
    ...
    [14] => Array
        (
            [date] => 2014-04-14
            [period] => 6
        )

    [15] => Array
        (
            [date] => 2014-04-21
            [period] => R1
        )

    [16] => Array
        (
            [date] => 2014-04-21
            [period] => R2
        )

    [17] => Array
        (
            [date] => 2014-04-21
            [period] => 4
        )

    [18] => Array
        (
            [date] => 2014-04-21
            [period] => 8
        )

    [19] => Array
        (
            [date] => 2014-04-28
            [period] => 1
        )
     ...
)

It has 2 types of sorts: Date and Period. But in the same time, the sort of the Period item is numeric and alphabetic. Looking the example of day 2014-04-21:

    [15] => Array
        (
            [date] => 2014-04-21
            [period] => R1
        )

    [16] => Array
        (
            [date] => 2014-04-21
            [period] => R2
        )

    [17] => Array
        (
            [date] => 2014-04-21
            [period] => 4
        )

    [18] => Array
        (
            [date] => 2014-04-21
            [period] => 8
        )

Is it possible to sort numerically and later alphabetically? I need a similar result:

[15] => Array
    (
        [date] => 2014-04-21
        [period] => 4
    )

[16] => Array
    (
        [date] => 2014-04-21
        [period] => 8
    )
[17] => Array
    (
        [date] => 2014-04-21
        [period] => R1
    )

[18] => Array
    (
        [date] => 2014-04-21
        [period] => R2
    )

I use this code:

function cmp($a, $b) {
    if ($a['date'] == $b['date']) { 
        if(is_numeric($a['period']) && !is_numeric($b['period'])) {
            return 1;
        }     
        else if(!is_numeric($a['period']) && is_numeric($b['period'])) {
            return -1;
        }
        else {
            return ($a['period'] < $b['period']) ? -1 : 1;
        }       
    }
    return (strtotime($a['date']) < strtotime($b['date']))? -1 : 1;
}

usort($arr, "cmp");