private_properties1.php

<?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'

private_properties2.php

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

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