有一个这样的数组，如何遍历出每级别下面的username数量，级别有n级

Array(
[cn103025] => Array
(
[username] => cn103025
[tjrname] => cn329150
[child] => Array
(
[cn220789] => Array
(
[username] => cn220789
[tjrname] => cn103025
)

            )

    )

[cn189165] => Array
    (
        [username] => cn189165
        [tjrname] => cn329150
    )

[cn959602] => Array
    (
        [username] => cn959602
        [tjrname] => cn329150
        [child] => Array
            (
                [cn686206] => Array
                    (
                        [username] => cn686206
                        [tjrname] => cn959602
                    )

            )

    )

)