PHP foreach循环真奇妙

奇妙一

for ($y = 0; $y <= 2; $y++) {
    foreach ($list as $index => &$age) {
        $age++;
        if ($age == 2) {
            $list[] = 0;
        }
    }
    dump($list);
}

奇妙二

请问页面能输出什么?

$list = [0];
foreach ($list as $key => $value) {
    $list[] = microtime(true);
    echo $value.PHP_EOL;
}
echo $value.PHP_EOL;
die;

奇妙三

请问能输出什么?

$list = [0];
foreach ($list as $key => &$value) {
    $list[] = microtime(true);
    echo $value.PHP_EOL;
}
echo $value.PHP_EOL;
die;

奇妙四

$list = [0];
for ($y=1; $y<=2 ;$y++) {
    for ($i = 0; $i < count($list); $i++) {
        $list[$i]++;

        if ($list[$i] == 2) {
            $list[] = 0;
        }
    }

    dump($list);
}

file