Shell Sort in PHP

<?php

class ShellSort {
   public static function sort(&$arr) {
       $arrLen = count($arr);
       $gap = 0;
       while($gap < (int)($arrLen / 3)) {
           $gap = $gap * 3 + 1;
       }

       while ($gap > 0) {
           for($i = $gap; $i < $arrLen; $i++) {
               $j = $i;
               $tmp = $arr[$i];
               while($j >= $gap && $arr[$j - $gap] > $tmp) {
                   $arr[$j] = $arr[$j - $gap];
                   $j = $j - $gap;
               }
               $arr[$j] = $tmp;
           }
           $gap = (int)(($gap - 1) / 3);
       }
       
   }
}