Assigning NULL to a PHP variable doesn't clean the memory

Question

I have read several times that, in order to invoke the garbage collector and actually clean the RAM used by a variable, you have to assign a new value (e.g. NULL) instead of simply unset() it.

This code, however, shows that the memory allocated for the array $a is not cleaned after the NULL assignment.

function build_array()
{
 for ($i=0;$i<10000;$i++){
    $a[$i]=$i;
 }
 $i=null;
 return $a;
}

echo '<p>'.memory_get_usage(true);

$a = build_array();

echo '<p>'.memory_get_usage(true);

$a = null;

echo '<p>'.memory_get_usage(true);

The output I get is:

So part of the memory is cleaned, but not all the memory. How can I completely clean the RAM?

Assigning `NULL` a variable isn't the same with using `unset()`. — Shafizadeh, Mar 09 '16 at 15:46
This is exactly what I said, I have used the assignment because unset isn't enough, but it is not working as expected. — Eugenio, Mar 09 '16 at 15:47
Try yourself to add unset(), it doesn't affect the result... — Eugenio, Mar 09 '16 at 15:48
Possible duplicate of [What's better at freeing memory with PHP: unset() or $var = null](http://stackoverflow.com/questions/584960/whats-better-at-freeing-memory-with-php-unset-or-var-null) — kerwan, Mar 09 '16 at 15:49
[The result after and before executing that function are identical using `unset()` function](https://3v4l.org/iRuRZ) — Shafizadeh, Mar 09 '16 at 15:54

score 4 · Answer 1 · answered Mar 09 '16 at 15:50

4

You have no way to definitely clear a variable from the memory with PHP.

Its is up to PHP garbage collector to do that when it sees that it should.

Fortunately, PHP garbage collector may not be perfect but it is one of the best features of PHP. If you do things as per the PHP documentation there's no reasons to have problems.

If you have a realistic scenario where it may be a problem, post the scenario here or report it to PHP core team.

Other unset() is the best way to clear vars.

answered Mar 09 '16 at 15:50

pipblip

61
3

Thanks, so basically the fact that assigning a value clean the memory in use, as I've read several times (e.g. here http://v1.srcnix.com/2010/02/10/7-tips-to-prevent-php-running-out-of-memory/) is not correct? – Eugenio Mar 09 '16 at 16:51

score 0 · Answer 2 · edited May 23 '17 at 11:50

0

Point is that memory_get_usage(true) shows the memory allocated to your PHP process, not the amount actually in use. System could free unused part once it is required somewhere. More details on memory_get_usage could be found there

If you run that with memory_get_usage(false), you will see that array was actually gc'd. example

edited May 23 '17 at 11:50

Community

1
1

answered Mar 09 '16 at 15:55

Evgeny Soynov

704
3
13

Yes, but finally the memory allocated is the important value I guess when it comes to avoid max memory allocation errors in PHP. – Eugenio Mar 09 '16 at 16:51

Assigning NULL to a PHP variable doesn't clean the memory

2 Answers2