variable array name

I seem to be having a problem in assigning a value to an array where the
array is called dynamically.

e.g. the physical name for the array is "my_array", so:
my_array[1] = "test";
works fine.

$array_name = "my_array";
$array_name[1] = "test";
does not work.

I have tried $$array_name[1] = "test"; but to no avail.

Any Ideas?

Cheers,

Mark
.