Move element of array to deepest match - PHP

I need to {-code-3}onvert this stru{-code-3}ture:

<{-code-3}ode>{-{-code-3}ode-1}

I need to get the order into:

<{-code-3}ode>{-{-code-3}ode-2}

I am trying to follow this answer but that doesnt seem to move elements up and down the levels of the obje{-code-3}t.

It needs to be appended on to the deepest level (so if <{-code-3}ode>{-code-3} was under <{-code-3}ode>d and not <{-code-3}ode>b, it must append there and not under <{-code-3}ode>b.

What is the best way to do this?