usort doesn't work

[metal3d@…]

Tried:

$objects = array();
for ($i=0; $i<10; $i++){
   $ob = new stdClass;
   $ob->data = uniqid();
   $objects[] = $ob;
}

function cmp($a,$b){
   return $b->data - $a->data
}
echo "start...";
usort($objects,"cmp");
echo "done \n";

"start" appears... and nothing appends.