Sort an Class objects array with fields

[Anon]

e.g.
class A{
var a;
var b;
var c;
..
}

and I declare an array of objects A
for (..){
$obj[] = new A();
..
}

I want to sort objects $obj[] according to
ascending order of a then ascending order of b then descending order of c

how to do with it?
thx