/* create the two entities, objects here */
$a = new ObjA;
$b = new ObjB;

/* create the circular reference between them */
$a->b = $b;
$b->a = $a;

/* Decrement refcount of both entities */
unset($a, $b);

/* Here, there is no way to fetch the two zvals from PHP
land, but as they own each other, they are still present in
memory and PHP will never free them, before PHP 5.3 */