如何实现洗牌算法

题目描述：

要求开发一款扑克游戏，编写一套洗牌算法，而公平地洗牌是将洗好的牌存储在一个整型数组里。

实现代码如下：

<?php
$cardNum = 54;

print_r(washCard(54));

function washCard($cardNum){
    $result = $tmp = [];
    //生成0~53的array
    $tmp = range(0,$cardNum-1);
    for($i = 0; $i < $cardNum; $i++){
        //随机找一个值
        $index = rand(0, $cardNum - $i - 1);
        //值赋给$result
        $result[$i] = $tmp[$index];
        //去掉
        unset($tmp[$index]);
        //格式化数组，key从0开始递增1
        $tmp = array_values($tmp);
    }

    return $result;
}

洗牌的结果是随机不唯一的，我的是。

Array
(
    [0] => 39
    [1] => 27
    [2] => 18
    [3] => 44
    [4] => 47
    [5] => 4
    [6] => 52
    [7] => 50
    [8] => 16
    [9] => 15
    [10] => 12
    [11] => 7
    [12] => 13
    [13] => 41
    [14] => 46
    [15] => 5
    [16] => 49
    [17] => 26
    [18] => 10
    [19] => 19
    [20] => 43
    [21] => 8
    [22] => 9
    [23] => 40
    [24] => 48
    [25] => 17
    [26] => 34
    [27] => 45
    [28] => 53
    [29] => 37
    [30] => 2
    [31] => 25
    [32] => 11
    [33] => 0
    [34] => 30
    [35] => 32
    [36] => 33
    [37] => 51
    [38] => 14
    [39] => 36
    [40] => 22
    [41] => 28
    [42] => 31
    [43] => 35
    [44] => 1
    [45] => 6
    [46] => 24
    [47] => 3
    [48] => 21
    [49] => 23
    [50] => 42
    [51] => 38
    [52] => 29
    [53] => 20
)