在 foreach 循环中什么更好...使用 &符号或基于键重新分配?

But after you've written your first 100 loops with references, do you honestly believe you won't have forgotten to unset a single reference? Of course not! It's so uncommon to unset variables that have been used in a loop (we assume the GC will take care of it for us), so most of the time, you don't bother. When references are involved, this is a source of frustration, mysterious bug-reports, or traveling values, where you're using complex nested loops, possibly with multiple references... The horror, the horror.
Besides, as time passes, who's to say that the next person working on your code won't foget about unset? Who knows, he might not even know about references, or see your numerous unset calls and deem them redundant, a sign of your being paranoid, and delete them all together. Comments alone won't help you: they need to be read, and everyone working with your code should be thoroughly briefed, perhaps have them read a full article on the subject. The examples listed in the linked article are bad, but I've seen worse, still:

foreach($nestedArr as &$array)
{
    if (count($array)%2 === 0)
    {
        foreach($array as &$value)
        {//pointless, but you get the idea...
            $value = array($value, 'Part of even-length array');
        }
        //$value now references the last index of $array
    }
    else
    {
        $value = array_pop($array);//assigns new value to var that might be a reference!
        $value = is_numeric($value) ? $value/2 : null;
        array_push($array, $value);//congrats, X-references ==> traveling value!
    }
}

这是一个简单的旅行值问题示例.我没有编造这个，顺便说一句，我遇到了归结为这个的代码......老实说.除了发现错误和理解代码(参考文献变得更加困难)之外，在这个例子中仍然很明显，主要是因为它只有 15 行长，即使使用宽敞的 Allman 编码风格......现在想象一下在代码中使用的这个基本结构实际上做一些更复杂、更有意义的事情.祝调试成功.

function recursiveFunc($n, $max = 10)
{
    if (--$max)
    {
        return $n === 1 ? 10-$max : recursiveFunc($n%2 ? ($n*3)+1 : $n/2, $max);
    }
    return null;
}
$array = range(10,20);
foreach($array as $k => $v)
{
    $v = recursiveFunc($v);//reassigning $v here
    if ($v !== null)
    {
        $array[$k] = $v;//only now, will the actual array change
    }
}
echo json_encode($array);

[7,11,12,13,14,15,5,17,18,19,8]

$array = range(10,20);
foreach($array as &$v)
{
    $v = recursiveFunc($v);//Changes the original array...
    //granted, if your version permits it, you'd probably do:
    $v = recursiveFunc($v) ?: $v;
}
echo json_encode($array);
//[7,null,null,null,null,null,5,null,null,null,8]

foreach($array as &$v)
{
    $temp = recursiveFunc($v);//creating copy here, anyway
    $v = $temp ? $temp : $v;//assignment doesn't require the lookup, though
}
//or:
foreach($array as &$v)
{
    $v = recursiveFunc($v) ? recursiveFunc($v) : $v;//2 calls === twice the overhead!
}
//or
$base = reset($array);//get the base value
foreach($array as $k => &$v)
{//silly combine both methods to fix what needn't be a problem to begin with
    $v = recursiveFunc($v);
    if ($v === 0)
    {
        $v = $base + $k;
    }
}