function subval_sort($a,$subkey)
  {
    foreach($a as $k=>$v)
    {
      $b[$k] = strtolower($v[$subkey]);
    }
    arsort($b);
    foreach($b as $key=>$val)
    {
      $c[] = $a[$key];
    }
    return $c;
  }

Array subval sort

php, sort, array

PHP