Heap Sort in PHP

<?php

class HeapSort {

   public static function sort(&$arr) {
       $size = count($arr);
       for($i = (int)($size / 2) - 1; $i >=0; $i--) {
           HeapSort::heapify($arr, $size, $i);
       }

       $j = $size - 1;
       while($j >= 1) {
           // Swap the first with arr[i].
           HeapSort::swap($arr, 0, $i);
           // Heapify the array again.
           HeapSort::heapify($arr, $i, 0);
           // Decrement i.
           $j--;
       }
   }

   public static function heapify(&$arr, $size, $i) {
       
       $largest = $i;
       $leftLeaf = $i *2 + 1;
       $rightLeaf = $i * 2 + 2;

       // If the left child is larger than the largest.
       if($leftLeaf < $size && $arr[$leftLeaf] > $arr[$largest]) {
           $largest = $leftLeaf;
       }
       // If the right child is larger than the largest.
       if($rightLeaf < $size && $arr[$rightLeaf] > $arr[$largest]) {
           $largest = $rightLeaf;
       }

       if($largest != $i) {
           // swap i and the largest.
           HeapSort::swap($arr, $i, $largest);
           // Heapify the subtree.
           HeapSort::heapify($arr, $size, $largest);
       }
   }

   public static function swap(&$arr, $a, $b) {
       $tmp = $arr[$a];
       $arr[$a] = $arr[$b];
       $arr[$b] = $tmp;
   }
}