PHP equivalent of friend or internal

PHP doesn't support any friend-like declarations. It's possible to simulate this using the PHP5 __get and __set methods and inspecting a backtrace for only the allowed friend classes, although the code to do it is kind of clumsy.

There's some sample code and discussion on the topic on PHP's site:

class HasFriends
{
    private $__friends = array('MyFriend', 'OtherFriend');

    public function __get($key)
    {
        $trace = debug_backtrace();
        if(isset($trace[1]['class']) && in_array($trace[1]['class'], $this->__friends)) {
            return $this->$key;
        }

        // normal __get() code here

        trigger_error('Cannot access private property ' . __CLASS__ . '::$' . $key, E_USER_ERROR);
    }

    public function __set($key, $value)
    {
        $trace = debug_backtrace();
        if(isset($trace[1]['class']) && in_array($trace[1]['class'], $this->__friends)) {
            return $this->$key = $value;
        }

        // normal __set() code here

        trigger_error('Cannot access private property ' . __CLASS__ . '::$' . $key, E_USER_ERROR);
    }
}

(Code proved by tsteiner at nerdclub dot net on bugs.php.net)

It is also possible to elevate privileges, aka leaking data selectively, using a handshake and closures in php >=5.3.3.

Basically, the interaction goes: class A has a public method which accepts a class B object, and calls B->grantAccess (or whatever your interface defines), passing it a closure. The closure use($that,$anythingelseyouneed) where $that=$this, and anything else you need to determine what properties are allowed to be accessed. The closure has one argument - the property to return; if it is a property on $that and everything is cool, the closure returns the property. Otherwise, it returns '', or throws an exception, or maybe a default value.

Class B->grantAccess accepts a callable and stores it, using it in other methods to pluck out private properties the closure allows to be leaked. Make class B's default constructor private. Construct a B using a static factory method that takes a Class A argument, to ensure the handshake happens.

Gist here: https://gist.github.com/mcamiano/00592fb400e5043d8acd