Sorting within Class

[griffinmt]

Having trouble using usort within a class.
Example:
class User {
var $fn;
var $ln;
var $id;
function User() { ...}
...
}
class Users {
var $list = array();
function Users() {...}
function addUser($U) {array_push($this->list,$U);}
...
}

above are two classes, the second (Users) is an array of User class objects. From within Users, I would like to sort the array (using usort) such that the elements are in sequence by User->ln

Cannot get it to invoke the comparison function and the result is a somewhat r****me sequence.

What is the most efficient way to call the usort and code the associated coomparision function?

Thanks