I need to transform one array into another.
The first array describes a message. The message has its own ID and the ID of its parent.
$firstArray[id]["parent_id"] = parent_id
I need to transform this into an array which describes which parent messages have which
children:
$secondArray(parent_id) = array(id, id, id, id);
So, for example:
$firstArray[4]["parent_id"] = 2;
$firstArray[6]["parent_id"] = 2;
$firstArray[12]["parent_id"] = 6;
$firstArray[13]["parent_id"] = 12;
into
$secondArray[12] = array(13);
$secondArray[6] = array(12);
$secondArray[2] = array(4, 6);