被引用的访问类？

I know this may seem like a strange question, but say I have two classes;

class Class1 {
    private $foo;
    function addBar(Class2 &$bar)
    {
        $this->foo = $bar;
    }
}


class Class2 {
    private $foo
}

$a = new Class1;
$b = new Class2;
$a->addBar($b);

Is there any way Class2 can read information from Class1? Even though they're in the same code block here, they are in seperate files in my script. The reason I need this is because I have Class2 linked by reference, but Class2 isn't allowed to be run until Class1 has been, and I know I could pass another variable to reference Class2 to Class1 (eg. $b->addFoo($a)), but I would rather avoid that if possible.

I hope this can be done!
Thanks in advance

EDIT

Thanks to everyone who helped, I have decided to go another way around it. Thankyou again.

Class2 can call methods on Class1 objects, though it can't access private data. But it must have a reference to a Class1 instance in order to call any methods on it.

Your example doesn't show Class2 actually containing any code, and it sounds like you're trying to avoid calling methods on $b. That doesn't make sense. Class2 can't "do" anything if you never call any methods on it.

class Class1 {
    private $foo;
    function addBar(Class2 $bar)
    {
        $this->foo = $bar;
        $bar->addBar($this);
    }
}

class Class2 {
    private $foo
    function addBar(Class1 $bar)
    {
        $this->foo = $bar;
    }
}

$a = new Class1;
$b = new Class2;
$a->addBar($b);

class Class1 {
    private $foo;
    function addBar(Class2 &$bar)
    {
        $this->foo = $bar;
        $bar->doSomething();
    }
}


class Class2 {
    private $foo
    function doSomething()
    {
        //code
    }
}

$a = new Class1;
$b = new Class2;
$a->addBar($b);

this way doSomething in Class2() is run after addBar() in Class1