Reference Counting in php - how does it work? - php

Question

I am trying to understand this article "PHP Manual -> Features -> Garbage Collection"
unfortunately few things are unclear for me.
1.
To avoid having to call the checking of garbage cycles with every
possible decrease of a refcount, the algorithm instead puts all
possible roots (zvals) in the "root buffer".
but what in case
<?php
$a = new \stdClass(); (1)
$a = new \stdClass();
Then I guess the first object become "lost" zval like
no_symbol : (refcount=1, is_ref=1) = stdObject
Will such "lost" zvals be added into root buffer or not? There is no handler for them.
2.
Variables created in function scope, what happened with them?
Ex:
<?php
function test($a = 'abc') {
$c = 123;
return 1;
}
test();
echo 'end';
What happened with $a and $c when gc starts?
These variables still have refcount set to 1.
Will they still be removed? if yes then why and how (what is happening under the cover?)
3.
How can it help for cyclic references?
Ex
<?php
$a = array('abc');
$a[] =& $a;
unset($a);
where
(refcount=1, is_ref=1)=array (
0 => (refcount=1, is_ref=0)='abc',
1 => (refcount=1, is_ref=1)=...
)

Answer 1

1) The original object is replaced with the new one, and thus memory is freed instantly.
echo memory_get_usage().'<br/>';
$a = new stdClass();
echo memory_get_usage().'<br/>';
$a = new stdClass();
echo memory_get_usage().'<br/>';
2) They are gc'ed the second the function completes executing:
echo memory_get_usage().'<br/>';
function test($a = 'abc') {
$c = 123;
return 1;
}
echo memory_get_usage().'<br/>';
test();
echo memory_get_usage().'<br/>';
3) unsetting $a will leave the referenced variable in memory. You need to set the value to NULL first, and then unset.
echo memory_get_usage().'<br/>';
$a = array('abc');
echo memory_get_usage().'<br/>';
$a[] =& $a;
echo memory_get_usage().'<br/>';
$a = null;
unset($a);
echo memory_get_usage().'<br/>';