在2009年的unset手册页面中提到:

unset() does just what its name says - unset a variable. It does not force immediate memory freeing. PHP's garbage collector will do it when it see fits - by intention as soon, as those CPU cycles aren't needed anyway, or as late as before the script would run out of memory, whatever occurs first. If you are doing $whatever = null; then you are rewriting variable's data. You might get memory freed / shrunk faster, but it may steal CPU cycles from the code that truly needs them sooner, resulting in a longer overall execution time.

(自2013年起，未设置的手册页不再包括该部分)

注意，在php5.3之前，如果循环引用中有两个对象，例如在父子关系中，在父对象上调用unset()将不会释放用于子对象中父引用的内存。当父对象被垃圾回收时，内存也不会被释放。

问题“unset和= null之间的差异”详细说明了一些差异:

Unset ($a)也从符号表中删除$a;例如:

$a = str_repeat('hello world ', 100);
unset($a);
var_dump($a);

输出:

Notice: Undefined variable: a in xxx
NULL

但是当使用$a = null时:

$a = str_repeat('hello world ', 100);
$a = null;
var_dump($a);

输出:

NULL

$a = null似乎比它的unset()对手快一点:更新符号表项似乎比删除它快。

当您尝试使用不存在(未设置)变量时，将触发一个错误，变量表达式的值将为null。(因为，PHP还应该做什么?每个表达式都需要产生某个值。) 一个赋值为null的变量仍然是一个完全正常的变量。

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。

郑重声明，不包括花费的时间:

<?php
echo "<hr>First:<br>";
$x = str_repeat('x', 80000);
echo memory_get_usage() . "<br>\n";      
echo memory_get_peak_usage() . "<br>\n"; 
echo "<hr>Unset:<br>";
unset($x);
$x = str_repeat('x', 80000);
echo memory_get_usage() . "<br>\n";      
echo memory_get_peak_usage() . "<br>\n"; 
echo "<hr>Null:<br>";
$x=null;
$x = str_repeat('x', 80000);
echo memory_get_usage() . "<br>\n";      
echo memory_get_peak_usage() . "<br>\n";

echo "<hr>function:<br>";
function test() {
    $x = str_repeat('x', 80000);
}
echo memory_get_usage() . "<br>\n";      
echo memory_get_peak_usage() . "<br>\n"; 

echo "<hr>Reasign:<br>";
$x = str_repeat('x', 80000);
echo memory_get_usage() . "<br>\n";      
echo memory_get_peak_usage() . "<br>\n"; 

它返回

First:
438296
438352
Unset:
438296
438352
Null:
438296
438352
function:
438296
438352
Reasign:
438296
520216 <-- double usage.

结论，空内存和未设置空闲内存都是预期的(不仅在执行结束时)。另外，对变量重新赋值会在某个点上将值保存两次(520216对438352)

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

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

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)