<?php
class Foo {
    public $bar;
    public $x = array(1,2,3);
    function __destruct() {
        if ($this->bar !== null) {
            $this->x = null;
            unset($this->bar);
        }
    }
}
class Bar {
    public $foo;
        function __destruct() {
                if ($this->foo !== null) {
                        unset($this->foo);
                }
        }

}
$foo = new Foo();
$bar = new Bar();
$foo->bar = $bar;
$bar->foo = $foo;
unset($foo);
unset($bar);
var_dump(gc_collect_cycles());
?>