Accessing private properties from other instances

In PHP, when a property or method is marked private, it can only be accessed from within that class. That includes other instances of the same class. This may seem counter-intuitive at first, because we are used to dealing with instances of classes. The visibility operator however works not on object-level, but on class level.

An example:

<?php
class Foo
{
    private $private;
    public function __construct($value)
    {
        $this->private = $value;
    }
    public function getOther(Foo $object)
    {
       return $object->private;
    }
}

$foo1 = new Foo('foo1');
$foo2 = new Foo('foo2');

echo $foo1->getOther($foo2); // outputs 'foo2'

This should make it clear that both instances of Foo have access to each other’s private properties.

What practical use does this have? A great candidate for this are Value Objects. If we want to make sure that to separate instances of Foo are actually equal, we can easily compare their private properties:

<?php
class Foo
{
    // ...
    public function equals(Foo $other)
    {
        return $this->private === $other->private;
    }
}

// ...
echo $foo1->equals($foo2) ? 'Equal' : 'Different';

Comments

Nicholas K. Dionysopoulos - 2011/03/24

Nice info, Mathias! It’s very counter-intuitive indeed. On the other OO languages I know of (Delphi, various .NET dialects) a private variable is inaccessible outside the context of the owner object instance.

Jonathan Mayhak - 2011/03/24

Good info. Had no idea php worked like this.

Also, great use case….very practical!

Lolol - 2011/03/24

This is also the case in Java, which i found rather awkward when i found it.. but apparently it has some use. kind of takes away the principle behind private if you can extend a class to create functions to read another instance of that base class` privates :)