对于对象，特别是在惰性加载场景中，应该考虑垃圾收集器运行在空闲的CPU周期中，因此假设您遇到了麻烦，当大量对象加载时，小时间惩罚将解决内存释放问题。

使用time_nanosleep使GC能够收集内存。 将变量设置为null是可取的。

在生产服务器上测试，最初作业消耗50MB，然后停止。 在使用nanosleep之后，14MB的内存消耗是不变的。

有人会说，这取决于GC行为，而GC行为可能会随着PHP版本的不同而变化。 但它在PHP 5.3上运行良好。

如。此示例(代码取自VirtueMart2谷歌提要)

for($n=0; $n<count($ids); $n++)
{
    //unset($product); //usefull for arrays
    $product = null
    if( $n % 50 == 0 )
    {
        // let GC do the memory job
        //echo "<mem>" . memory_get_usage() . "</mem>";//$ids[$n];
        time_nanosleep(0, 10000000);
    }

    $product = $productModel->getProductSingle((int)$ids[$n],true, true, true);
    ...

对于通过引用复制的变量，它的工作方式不同:

$a = 5;
$b = &$a;
unset($b); // just say $b should not point to any variable
print $a; // 5

$a = 5;
$b = &$a;
$b = null; // rewrites value of $b (and $a)
print $a; // nothing, because $a = null

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。

PHP 7已经着手解决这类内存管理问题，并将其使用减少到最小。

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

  echo "took $elapsed seconds\r\n";

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

  echo "took $elapsed seconds\r\n";

?>

PHP 7.1输出:

花了0.16778993606567秒 花了0.16630101203918秒

对于对象，特别是在惰性加载场景中，应该考虑垃圾收集器运行在空闲的CPU周期中，因此假设您遇到了麻烦，当大量对象加载时，小时间惩罚将解决内存释放问题。

使用time_nanosleep使GC能够收集内存。 将变量设置为null是可取的。

在生产服务器上测试，最初作业消耗50MB，然后停止。 在使用nanosleep之后，14MB的内存消耗是不变的。

有人会说，这取决于GC行为，而GC行为可能会随着PHP版本的不同而变化。 但它在PHP 5.3上运行良好。

如。此示例(代码取自VirtueMart2谷歌提要)

for($n=0; $n<count($ids); $n++)
{
    //unset($product); //usefull for arrays
    $product = null
    if( $n % 50 == 0 )
    {
        // let GC do the memory job
        //echo "<mem>" . memory_get_usage() . "</mem>";//$ids[$n];
        time_nanosleep(0, 10000000);
    }

    $product = $productModel->getProductSingle((int)$ids[$n],true, true, true);
    ...

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