sort associative arrays in multidimensional arrays

Question

Is there a way to sort this array by the value of "displayName" key? I'm trying to understand the usort with no luck Any help would much appreciated

Array
(
    [totalCount] => 3
    [results] => Array
        (
            [0] => Array
                (
                    [id] => 2
                    [displayName] => LDN
                )

            [1] => Array
                (
                    [id] => 4
                    [displayName] => AMS
                )

            [2] => Array
                (
                    [id] => 5
                    [displayName] => CGN
                )

        )

)

Thank you in advance