Николай

$numbers = [1, 15, 3, 9800, 9, 76, 45, 9];
$arr = [];

$maxLength = max(array_map('strlen', $numbers));

foreach ($numbers as $number) {
    $key = str_pad((string) $number, $maxLength, $number);
   
    $arr[$key] .=$number;
  }

krsort($arr);

var_dump(implode('', $arr));