如何使用分治法在PHP中实现归并排序算法并提高排序效率？

function mergeSort($arr) {
    if (count($arr) <= 1) {
        return $arr;
    }
    $mid = floor(count($arr) / 2);
    $left = array_slice($arr, 0, $mid);
    $right = array_slice($arr, $mid);
    $left = mergeSort($left);
    $right = mergeSort($right);
    return merge($left, $right);
}

function merge($left, $right) {
    $result = [];
    while (count($left) > 0 && count($right) > 0) {
        if ($left[0] <= $right[0]) {
            $result[] = array_shift($left);
        } else {
            $result[] = array_shift($right);
        }
    }
    while (count($left) > 0) {
        $result[] = array_shift($left);
    }
    while (count($right) > 0) {
        $result[] = array_shift($right);
    }
    return $result;
}

$arr = [5, 3, 8, 6, 2, 9, 1, 7, 4];
$sortedArr = mergeSort($arr);
print_r($sortedArr);