Array
(
    [a1] => hello
    [a2] => hola
    [a3] => Array
        (
            [b1] => goodbye
            [b2] => cya
        )

)
<hr />Array
(
    [a1] => no
    [a3] => Array
        (
            [b1] => no
        )

    [a4] => added
)
<hr />Array
(
    [a1] => no
    [a2] => hola
    [a3] => Array
        (
            [b1] => no
            [b2] => cya
        )

    [a4] => added
)