venom, а так
function subval_sort($a,$subkey, $order='asc',$natural = true) {
if (count($a) != 0 || (!empty($a))) {
foreach($a as $k=>$v) {
$b[$k] = $v[$subkey];
}
if($natural){
natsort($b);
if($order=='desc') $b = array_reverse($b,true);
}
else {
($order=='asc')? asort($b) : arsort($b);
}
foreach($b as $key=>$val) {
$c[] = $a[$key];
}
return array_change_key_case(array_map('strtolower', $c));
}
}