How to sort a multidimensional array by firstName in php?

I have a multidimensional array that has arrays as elements, and inside those elements I have values (groupName, firstName, lastName, etc). I'm tyring to sort this multidimensional array by firstName. Here's my mutidimensional array:

Array
(
[Group1] => Array
    (
        [0] => Array
            (
                [groupName] => Group1
                [phoneNumber] => 13037777777
                [firstName] => Mike
                [lastName] => Jeff
            )

        [1] => Array
            (
                [groupName] => Group1
                [phoneNumber] => 13033422109
                [firstName] => Ben
                [lastName] => Morris
            )

    )

[Group2] => Array
    (
        [0] => Array
            (
                [groupName] => Group2
                [phoneNumber] => 13033213453
                [firstName] => Zumia
                [lastName] => Brown
            )

        [1] => Array
            (
                [groupName] => Group2
                [phoneNumber] => 13039899231
                [firstName] => Ana
                [lastName] => McLwius
            )

    )

[Group3] => Array
    (
        [0] => Array
            (
                [groupName] => Group3
                [phoneNumber] => 13030098342
                [firstName] => Junior
                [lastName] => White
            )

        [1] => Array
            (
                [groupName] => Group3
                [phoneNumber] => 13033222098
                [firstName] => Junior
                [lastName] => Smith
            )

    )

)

I know if it was a simple array (not a multidimensional array) I can just do:

   usort($peopleArray, function ($v1, $v2) {
        return strcmp($v1['firstName'], $v2['firstName']);
    });

Any ideas on how to accomplish this please? Thanks a lot in advance!

via Chebli Mohamed