Find all second level keys in multi-dimensional array in php

Question

I want to provide a list of the second-level keys that are employed. Not all of the keys in a record are the same. But I must be aware of the location of every key. array keys() just produces a list of numbers; it is not functional.

Basically, this is the output I'm after:

action, id, validate, Base, Ebase, Ftype, Qty, Type, Label, Unit, Validate, and

I have a sizable multidimensional array that has the following structure:

Array
(
    [0] => Array
        (
            [action] => A
            [id] => 1
            [validate] => yes
            [Base] => Array
                (
                    [id] => 2945
                )

            [EBase] => Array
                (
                    [id] => 398
                )

            [Qty] => 1
            [Type] => Array
                (
                    [id] => 12027
                )

            [Label] => asfhjaflksdkfhalsdfasdfasdf
            [Unit] => asdfas
        )

    [1] => Array
        (
            [action] => A
            [id] => 2
            [validate] => yes
            [Base] => Array
                (
                    [id] => 1986
                )

            [FType] => Array
                (
                    [id] => 6
                )

            [Qty] => 1
            [Type] => Array
                (
                    [id] => 13835
                )

            [Label] => asdssdasasdf
            [Unit] => asdger
        )
)

Can someone please help me with this?