PHP 如何为变量分配和释放内存？

Question

我想知道 PHP 何时释放用于变量的内存，

例如

function foo(){
  $foo = 'data';
  return $foo;  // <- is the memory space for `$foo` emptied at this point?
}

它是否比以下速度慢

function foo(){
  return 'data'; 
}

？

Answer 1

好吧，让我们来看看吧！

<?php
$checkpoints = array( 'start' => memory_get_usage() );

$checkpoints['before defining demo1'] = memory_get_usage();
function demo1() { $foo = 'data'; return $foo; }
$checkpoints['after defining demo1'] = memory_get_usage();

$checkpoints['before defining demo2'] = memory_get_usage();
function demo2() { return 'data'; }
$checkpoints['after defining demo2'] = memory_get_usage();


$checkpoints['before calling demo1'] = memory_get_usage();
demo1();
$checkpoints['after calling demo1'] = memory_get_usage();

$checkpoints['before calling demo2'] = memory_get_usage();
demo2();
$checkpoints['after calling demo2'] = memory_get_usage();

$checkpoints['before calling demo1 with storage'] = memory_get_usage();
$storage1 = demo1();
$checkpoints['after calling demo1 with storage'] = memory_get_usage();

$checkpoints['before calling demo2 with storage'] = memory_get_usage();
$storage2 = demo2();
$checkpoints['after calling demo2 with storage'] = memory_get_usage();

echo '<pre>';
print_r($checkpoints);

$last_key = 'start';
foreach($checkpoints as $key => $value) {
    echo "{$key} - {$last_key} = ", ($value - $checkpoints[$last_key]), "\n";
    $last_key = $key;
}

在 PHP 5.3.6 下，我的输出是：

Array
(
    [start] => 321920
    [before defining demo1] => 322188
    [after defining demo1] => 322788
    [before defining demo2] => 322880
    [after defining demo2] => 323188
    [before calling demo1] => 323280
    [after calling demo1] => 323368
    [before calling demo2] => 323464
    [after calling demo2] => 323552
    [before calling demo1 with storage] => 323692
    [after calling demo1 with storage] => 323896
    [before calling demo2 with storage] => 324000
    [after calling demo2 with storage] => 324204
)

然后

start - start = 0
before defining demo1 - start = 268
after defining demo1 - before defining demo1 = 600
before defining demo2 - after defining demo1 = 92
after defining demo2 - before defining demo2 = 308
before calling demo1 - after defining demo2 = 92
after calling demo1 - before calling demo1 = 88
before calling demo2 - after calling demo1 = 96
after calling demo2 - before calling demo2 = 88
before calling demo1 with storage - after calling demo2 = 140
after calling demo1 with storage - before calling demo1 with storage = 204
before calling demo2 with storage - after calling demo1 with storage = 104
after calling demo2 with storage - before calling demo2 with storage = 204

在丢弃输出的 demo1 和 demo2 的初始调用期间内存的增加很可能是由于创建造成的存储内存使用的变量。

然而，这里的底线是两个存储示例，其中直接返回数据并在返回数据之前将其分配给变量导致给定数据使用完全相同的内存。

结论：在这个简单的测试中，PHP 似乎足够聪明，不会不必要地复制字符串变量——尽管请注意两个函数之间的内存使用差异。仅声明 demo1 函数就比声明 demo2 占用更多内存。确实有几百个字节。