<?php
class Foo {
    public $a;
    function __destruct() {
        echo "-> ";
        $a = array();
        $a[] =& $a;
        unset($a);
        var_dump(gc_collect_cycles());
    }
}
$a = new Foo();
$a->a = $a;
unset($a);
var_dump(gc_collect_cycles());
echo "ok\n"
?>