Groupping one of the value multi assoc array using another as group factor

I have this problem.
I must admit it has bested me - for now.

I have assoc multi-array passed by POST.

I would like to group [final_price] => value using [tax_class_id] => as grouping factor and possibly get results in an array.
Of course it has to be a dynamic process, as number of subarrays will change. Also [tax_class_id] => is not a contant and can be changed, deleted, added etc.

See sample below:
--------------------
1. What I have:
--------------------
Array (
[0 ] => Array (
[id] => 840
[model] => DMCFS20EB
[quantity] => 1
[final_price] => 166.8
[tax_class_id] => 0
)
[1] => Array (
[id] => 821
[model] => TH42PX80B
[quantity] => 1
[final_price] => 570.2042
[tax_class_id] => 7
)
[2] => Array (
[id] => 614
[model] => SL1000
[quantity] => 2
[final_price] => 215.99
[tax_class_id] => 8
)
[3] => Array (
[id] => 282
[model] => RDRHXD870
[quantity] => 1
[final_price] => 199.991395
[tax_class_id] => 7
)
)

-----------------------------
2. what I'd like to get:
-----------------------------
-> subarrays groupped by [tax_class_id] =>
-> [final_price] => added for same [tax_class_id]s

Array (
[1] => Array (
[tax_class_id] => 0
[final_price] => 166.8
)
[2] => Array (
[tax_class_id] => 7
[final_price] => 770.19559
)
[3] => Array (
[tax_class_id] => 8
[final_price] => 215.99
)
)

eventually [tax_class_id] can go into key:

Array (
[1] => Array (
[0] => 166.8
)
[2] => Array (
[7] => 770.19559
)
[3] => Array (
[8] => 215.99
)
)