PHP指针行为异常

在进行检查Laravel模型是否相互关联的项目时,我注意到PHP发生了一些(奇怪的？)指针行为.下面是一个重现我发现的最小示例.

<?PHP

$arr = ['a', 'b', ['c']];


foreach($arr as &$letter) {
    if (!is_array($letter)) {
        $letter = [$letter];    
    }
}

dump($arr);

foreach($arr as $letter) {
    dump($arr);
}

function dump(...$dump) {
    echo '<pre>';
    var_dump($dump);
    echo '</pre>';  
}

起初,我希望此响应中的转储全部返回相同的数据：

[ ['a'], ['b'], ['c'] ]

但是事实并非如此,我得到了以下回应：

[ ['a'], ['b'], ['c'] ]
[ ['a'], ['b'], ['a'] ]
[ ['a'], ['b'], ['b'] ]
[ ['a'], ['b'], ['b'] ]

可以找到一个正在运行的示例here.

指针为何如此运行？如何在第一个循环中更新$letter而不必执行$arr [$key] = $letter？

编辑：由于人们似乎误解了为什么还有第二个foreach循环,这是为了表明数组在变化而没有被重新分配

解决方法:

根据PHP documentation：

Reference of a $value and the last array element remain even after the foreach loop. It is recommended to destroy it by 07001.

$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
    $value = $value * 2;
}
// $arr is Now array(2, 4, 6, 8)

// Without an `unset($value)`, `$value` is still a reference to the last item: `$arr[3]`

foreach ($arr as $key => $value) {
    // $arr[3] will be updated with each value from $arr...
    echo "{$key} => {$value} ";
    print_r($arr);
}
// ...until ultimately the second-to-last value is copied onto the last value

/* output:
   0 => 2 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 2 )
   1 => 4 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 4 )
   2 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 )
   3 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 ) */