通过对变量执行unset()操作，您实际上已经将变量标记为“垃圾收集”(PHP实际上并没有这样的功能，但为了举例说明)，因此内存不能立即可用。该变量不再存储数据，但堆栈仍然保持较大的大小。执行null方法几乎会立即丢弃数据并缩小堆栈内存。

这是我个人的经验，也是别人的经验。在这里查看unset()函数的注释。

我个人在循环的迭代之间使用unset()，这样我就不必在大小上有堆栈的延迟。数据消失了，但足迹还在。在下一次迭代时，内存已经被php占用，因此可以更快地初始化下一个变量。

我仍然怀疑这一点，但我已经在我的脚本中尝试过，我正在使用xdebug来了解它将如何影响我的应用程序内存使用。 脚本是这样设置在我的函数:

function gen_table_data($serv, $coorp, $type, $showSql = FALSE, $table = 'ireg_idnts') {
    $sql = "SELECT COUNT(`operator`) `operator` FROM $table WHERE $serv = '$coorp'";
    if($showSql === FALSE) {
        $sql = mysql_query($sql) or die(mysql_error());
        $data = mysql_fetch_array($sql);
        return $data[0];
    } else echo $sql;
}

我在返回代码前添加了unset它给我160200 然后我尝试用$sql = NULL更改它，它给我:160224:)

但有一些独特的比较，当我不使用unset()或NULL, xdebug给我160144作为内存使用量

所以，我认为给line使用unset()或NULL将添加进程到您的应用程序，它将更好地保持原点与您的代码和减少变量，你正在使用尽可能有效。

如果我说错了请指正，谢谢

它对数组元素有影响。

考虑这个例子

$a = array('test' => 1);
$a['test'] = NULL;
echo "Key test ", array_key_exists('test', $a)? "exists": "does not exist";

这里，键“test”仍然存在。然而，在这个例子中

$a = array('test' => 1);
unset($a['test']);
echo "Key test ", array_key_exists('test', $a)? "exists": "does not exist";

密钥不再存在。

Unset实际上不是一个函数，而是一种语言结构。与其说它是一个函数调用，不如说它是一个return或include。

除了性能问题，使用unset可以使代码的意图更加清晰。

如果没有释放即时内存，则取消设置代码仍然是非常有用的，并且每次在退出方法之前传递代码步骤时都这样做是一个很好的实践。请注意，这不是关于释放即时内存。 直接内存是CPU的，辅助内存是RAM。

这也解决了防止内存泄漏的问题。

请参阅此链接 http://www.hackingwithphp.com/18/1/11/be-wary-of-garbage-collection-part-2

我使用unset已经很长时间了。

更好的做法是在代码中立即取消所有已经作为数组使用的变量。

$data['tesst']='';
$data['test2']='asdadsa';
....
nth.

只需要unset($data);释放所有变量的使用。

请参阅相关主题取消设置

在PHP中取消变量设置有多重要?

(错误)

来自注释的代码示例

echo "PHP Version: " . phpversion() . PHP_EOL . PHP_EOL;

$start = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    $a = 'a';
    $a = NULL;
}
$elapsed = microtime(true) - $start;

echo "took $elapsed seconds" . PHP_EOL;



$start = microtime(true);
for ($i = 0; $i < 10000000; $i++) {
    $a = 'a';
    unset($a);
}
$elapsed = microtime(true) - $start;

echo "took $elapsed seconds" . PHP_EOL;

在docker容器中运行image php:7.4-fpm等。

PHP Version: 7.4.8
took 0.22569918632507 seconds null
took 0.11705803871155 seconds unset
took 0.20791196823121 seconds null
took 0.11697316169739 seconds unset

PHP Version: 7.3.20
took 0.22086310386658 seconds null
took 0.11882591247559 seconds unset
took 0.21383500099182 seconds null
took 0.11916995048523 seconds unset

PHP Version: 7.2.32
took 0.24728178977966 seconds null
took 0.12719893455505 seconds unset
took 0.23839902877808 seconds null
took 0.12744522094727 seconds unset

PHP Version: 7.1.33
took 0.51380109786987 seconds null
took 0.50135898590088 seconds unset
took 0.50358104705811 seconds null
took 0.50115609169006 seconds unset

PHP Version: 7.0.33
took 0.50918698310852 seconds null
took 0.50490307807922 seconds unset
took 0.50227618217468 seconds null
took 0.50514912605286 seconds unset

PHP Version: 5.6.40
took 1.0063569545746 seconds null
took 1.6303179264069 seconds unset
took 1.0689589977264 seconds null
took 1.6382601261139 seconds unset

PHP Version: 5.4.45
took 1.0791940689087 seconds null
took 1.6308979988098 seconds unset
took 1.0029168128967 seconds null
took 1.6320278644562 seconds unset

但是，再举一个例子:

<?php
ini_set("memory_limit", "512M");

echo "PHP Version: " . phpversion() . PHP_EOL . PHP_EOL;

$start = microtime(true);
$arr = [];
for ($i = 0; $i < 1000000; $i++) {
    $arr[] = 'a';
}
$arr = null;
$elapsed = microtime(true) - $start;

echo "took $elapsed seconds" . PHP_EOL;



$start = microtime(true);
$arr = [];
for ($i = 0; $i < 1000000; $i++) {
    $arr[] = 'a';
}
unset($arr);
$elapsed = microtime(true) - $start;

echo "took $elapsed seconds" . PHP_EOL;

结果:

PHP Version: 7.4.8
took 0.053696155548096 seconds
took 0.053897857666016 seconds

PHP Version: 7.3.20
took 0.054572820663452 seconds
took 0.054342031478882 seconds

PHP Version: 7.2.32
took 0.05678391456604 seconds
took 0.057311058044434 seconds


PHP Version: 7.1.33
took 0.097366094589233 seconds
took 0.073100090026855 seconds

PHP Version: 7.0.33
took 0.076443910598755 seconds
took 0.077098846435547 seconds

PHP Version: 7.0.33
took 0.075634002685547 seconds
took 0.075317859649658 seconds

PHP Version: 5.6.40
took 0.29681086540222 seconds
took 0.28199100494385 seconds

PHP Version: 5.4.45
took 0.30513095855713 seconds
took 0.29265689849854 seconds