如何将数组的一个键的值添加到同一个数组的另一个键？

I have an array and I want to add value of one key of that array to another key of same array. This is my code

$current_user = $current_user->user_name;     
$_SESSION['user_name'] = $current_user;

Now I want to use this $_SESSION['user_name'] and add this value to another key of same array $_SESSION['_PREFERENCES']. This key has values in it. I want something like this

$_SESSION['(value of user_name key)_PREFERENCES']

Please guide me how to do it? Thank you.

Add concatenation:

$_SESSION[$_SESSION['user_name'] . '_PREFERENCES'] = $USER_PREFERENCES;
// May this variable be array or string.