Parsing FormData to just id

If I dump the results of a form post

$sel = collect(request('selected'));

I get the following:

The number of arrays and the number of subarrays may vary.

I need a way to grab the ids from a structure like this and get something like

[ 34, 35, 19]

I tried flatten, flatMap and map but was unsuccessful.

Any ideas appreciated.

via Chebli Mohamed