Does PHP take RAM when defining variable from variable and taking instance of object to another variable?

Question

In case I have this code

<?php
$array = array();
for ($i=1;$i<100000;$i++){
    $array[$i] = md5(rand(0,9999999999999999));
}

$array2 = $array;

$array takes about 0.5MB RAM, let's say. Does PHP proccess take about 1.0MB RAM with $array2 = $array; ? and in this case

<?php
class rand{
    public $array;
    function rand(){
        $this->array = array();
        for ($i=1;$i<100000;$i++){
            $this->array[$i] = md5(rand(0,9999999999999999));
        }

    }
}

$class = new rand();
$class2 = $class;

$class takes about 0.5MB RAM, let's say . Does PHP proccess take 1.0MB with $class2 = $class?

is it same?

Tests:

First
Second

@JohnCartwright: Wow. It looks it's not copying themselves. Look first, second — genesis
– genesis, Commented Jul 28, 2011 at 17:48
@genesis: You need to write another get_memory_usage() for the first variable, array or object. Your code doesn't do anything. — Cybercartel
– Cybercartel, Commented Jul 28, 2011 at 17:53
@Jitamaro: see my question edited. It is testing exact code I have in question — genesis
– genesis, Commented Jul 28, 2011 at 17:57

Wrikken · Accepted Answer · 2011-07-28 18:45:14Z

1

This is what the PHP manual in the reference section warns about: the Engine is smart enough. Setting the $array2 = $array; does not cause duplicate storage, as PHP recognizes they are still both the same. However, try a $array[2] = 'something;' after that. PHP detects the difference, and only then will copy the values.

<?php
$array = array();
for ($i=1;$i<100000;$i++){
    $array[$i] = md5(rand(0,9999999999999999));
}
echo memory_get_usage().PHP_EOL;
$array2 = $array;
echo memory_get_usage().PHP_EOL;
$array['foo'] = 'bar';
echo memory_get_usage().PHP_EOL;
//17252052
//17252156
//23776652

Classes are references by default, and only a clone $object would result in 2 objects in PHP >= 5.

edited Jul 28, 2011 at 18:45

answered Jul 28, 2011 at 18:31

Wrikken

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8 Comments

genesis Over a year ago

How is it possible that it takes a lot more memory for me? extensions?

Cybercartel Over a year ago

is there an option to kill this smart behaviour? I can compile myself!

Wrikken Over a year ago

@genesis: edited the classes answer in. And why it takes more? There's a random factor of course, but this is from a lean & mean command-line php script, not in a webserver, which is quite a difference.

Wrikken Over a year ago

@Jitamaro: the C source code of PHP is available, go ahead. I can't for the life of me think why you would destroy this behavior though. There are as of yet no ill sideeffects of this behavior that I know of.

Cybercartel Over a year ago

@genesis: you see you need 3 memory_get_usage!!! I want an upvote now for my post or I delete it!!!

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