Description:
------------
When freeing array variables with multiple references, variables which should still be accessible via other reference may be unexpectedly freed.

Depending of the code it may produce data corruption, glibc-detected memory corruption or segfault.

Reproduce code:
---------------
<?php

$root=array('id'=>'1', 'parent_id'=>NULL);
$child1 = array('id'=>'2', 'parent_id'=>'1');

// build tree with references up and down
$root['childs'] = array();

$child1['parent'] = & $root;
$child1['parent']['childs'][] = & $child1;
$child1['childs'] = array();


print_r($root['childs'][0]['childs']);

unset($child1);

print_r($root['childs'][0]['childs']);

?>


Expected result:
----------------
Array
(
)
Array
(
)



Actual result:
--------------
CGI:
Array
(
)
Array
(
)
Segmentation fault

Apache2/prefork/mod_php: no output, Segmentation fault in log