PHP 如何为变量分配和释放内存 - How does PHP assign and free memory for variables?

我想知道PHP什么时候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'; 
}

？

好吧，让我们找出答案！

<?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

在对丢弃输出的demo1和demo2的初始调用期间，内存增加很可能是由于创建了用于存储内存使用的变量。

但是，这里的底线是两个存储示例，其中直接返回数据并在返回之前将其分配给变量，导致给定数据的内存使用量完全相同。

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——尽管要注意两个函数之间的内存使用差异。仅声明 demo1 函数比声明demo2占用更多内存。几百字节，真的。