Differences between $var = null and unset($var)

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What's better at freeing memory with PHP: unset() or $var = null

As far as the garbage collection goes, is 1 better than the other in any circumstances?

EDIT: Particularly if $var is a very large variable with a lot of levels of recursion and other objects (so to do with recursive cleanup of large objects)

EDIT: Removed this: I can think of only 1 thing, and that's that isset($var) will respond differently in either case.

Because apparently I was mistaken... They both react the same.

Why do you need to unset it? What's wrong with automatic doing that as soon as variable is out of scope? — zerkms, Aug 02 '12 at 02:10
"is a very large variable with a lot of levels of recursion and other objects" -- so? — zerkms, Aug 02 '12 at 02:11
It's to do with weak references. The variable never actually goes "out of scope" per se, it just needs to be cleaned up when it becomes invalid. — Ricky Cook, Aug 02 '12 at 02:11
It's a fantastic addition... Highly needed for high performance apps. — Ricky Cook, Aug 02 '12 at 02:12
ok, it is extension. But back to the question: why do you need to unset the variable? Is there any issues with memory amount required by script? Is your script a "daemon"-like (the one that works continuously)? — zerkms, Aug 02 '12 at 02:13
No, but it provides a cache-like function. Basically, you load a whoooooole lot (millions) of objects, and you keep cache the most important using a weak reference... That way, if the variable is in scope somewhere, it keeps the reference in cache. If not, it's free to be collected. I need to delete the reference object when the contained var goes out of scope. — Ricky Cook, Aug 02 '12 at 02:15
@FuzziBear: reference is removed by php when variable goes out of scope. "if the variable is in scope somewhere" --- there is only one thread in php, so there is no "somewhere" — zerkms, Aug 02 '12 at 02:17
"and you keep cache the most important using a weak reference" --- if you want your GC to work as normal - don't use weakref extension. As simple as 1-2-3 ;-) — zerkms, Aug 02 '12 at 02:19
Whilst there's only 1 thread, you can have a very complex object hierarchy. If you load image data and store it in a class somewhere, you really only want to cache it for as long as it's already in memory (anything else is a waste). Weak references are REALLY important for very large data in very large scale applications. — Ricky Cook, Aug 02 '12 at 02:26
@Michael: whilst it's similar (and my use case could have been answered by that if I had have found it in my google search), I think there are more subtle differences like the reference and undefined notice behaviours that make this a question in and of itself. — Ricky Cook, Aug 02 '12 at 02:36

xdazz · Answer 1 · 2012-08-02T02:28:58.310

unset($var);
// You will get Undefined variable Notice.
if ($var) {}

$var = null;
// it's ok.
if ($var) {}

Addition of GC of php.

PHP's Garbage Collection is based on the refcount of the zval, if refcount becomes 0, then the zval can be freed.

So if $a = $b = array(/*a very large array*/);, unset only $a or only $b won't free the memory of the large array.

unset($a); or $a = null or assign another value to $a will all let the refcount decrease by 1, but the memory will be freed only when the refcount decrease to 0.

rtuner · Accepted Answer · 2012-08-15T02:29:27.230

3

unset does not force immediate memory freeing but leaves it for the gc. $var = null; however forces immediate memory release.

See example:

 // $a = NULL; (better I think)
 $a = 5;
 $b = & $a;
 $a = NULL;
 print "b $b "; // b 
 print(! isset($b)); // 1 
 ?>

It is also worthy to note that in the case of an array unset destroys the variable completely. i.e.:

<?php
$ar = array(1,2,3,4);
var_dump($ar);
echo "<br />";

unset($ar[2]);
var_dump($ar);
echo "<br />";

$ar[1] = null;
var_dump($ar);
?>

Returns the output:

array(4) { [0]=> int(1) [1]=> int(2) [2]=> int(3) [3]=> int(4) } 
array(3) { [0]=> int(1) [1]=> int(2) [3]=> int(4) } 
array(3) { [0]=> int(1) [1]=> NULL [3]=> int(4) }

edited Aug 15 '12 at 02:29

answered Aug 02 '12 at 02:26

rtuner

2,362
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I wish I could select multiple answers for the solution... Since I can only select 1, this is basically what I was looking for. For future readers, RiaD's and xdazz's answers also give very helpful information for other use cases. Xdazz really went the extra mile with explanation about GC internals ;) – Ricky Cook Aug 02 '12 at 02:32
How does that example demonstrate anything about "forcing" the garbage collector?! – deceze Jul 26 '17 at 15:39

score 2 · Answer 3 · answered Aug 02 '12 at 02:26

2

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


$a = 5;
$b = &$a;
$b = null;
print $a; // nothing, because $a = null

answered Aug 02 '12 at 02:26

RiaD

46,822
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Differences between $var = null and unset($var)

3 Answers3