function shell_sort(&$arr) {          if(!is_array($arr)) return;         $n = count($arr);         for($gap=floor($n/2);$gap>0;$gap=floor($gap/=2))         {                 for($i=$gap;$i<$n;++$i)                 {                         for($j=$i-$gap;$j>=0&&$arr[$j+$gap]<$arr[$j];$j-=$gap)                         {                                 $temp = $arr[$j];                                 $arr[$j] = $arr[$j+$gap];                                 $arr[$j+$gap] = $temp;

                    }

              }

        }

}