Differences between $var = null and unset($var) [duplicate]

Question

Possible Duplicate:
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.

Answer 1

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.

Answer 2

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) }