PHP中如何实现数组最大堆？

class MaxHeap {
    private $heap;

    public function __construct() {
        $this->heap = [];
    }

    public function insert($value) {
        $this->heap[] = $value;
        $this->heapifyUp(count($this->heap) - 1);
    }

    private function heapifyUp($index) {
        $parent = floor(($index - 1) / 2);
        while ($index > 0 && $this->heap[$parent] < $this->heap[$index]) {
            $this->swap($parent, $index);
            $index = $parent;
            $parent = floor(($index - 1) / 2);
        }
    }

    public function extractMax() {
        if (count($this->heap) == 0) {
            return null;
        }
        if (count($this->heap) == 1) {
            return array_pop($this->heap);
        }
        $max = $this->heap[0];
        $this->heap[0] = array_pop($this->heap);
        $this->heapifyDown(0);
        return $max;
    }

    private function heapifyDown($index) {
        $maxIndex = $index;
        $left = 2 * $index + 1;
        $right = 2 * $index + 2;

        if ($left < count($this->heap) && $this->heap[$left] > $this->heap[$maxIndex]) {
            $maxIndex = $left;
        }

        if ($right < count($this->heap) && $this->heap[$right] > $this->heap[$maxIndex]) {
            $maxIndex = $right;
        }

        if ($index != $maxIndex) {
            $this->swap($index, $maxIndex);
            $this->heapifyDown($maxIndex);
        }
    }

    private function swap($i, $j) {
        $temp = $this->heap[$i];
        $this->heap[$i] = $this->heap[$j];
        $this->heap[$j] = $temp;
    }
}