class A {
        public function foo(B $b) {
            $b->a = &$this;
            var_dump($b);
        }
    }

    class B {
        public $a = null;
    }

    $a = new A;
    $b = new B;
    $a->foo($b);
    var_dump($b);

// Output: object(B)#2 (1) { ["a"]=>  &object(A)#1 (0) { } } object(B)#2 (1) { ["a"]=>  object(A)#1 (0) { } }