private.php

<?php
class ParentClass
{
    private $privateProperty = 'inParent';

    public function __get($name)
    {
      if (property_exists(get_class($this), $name)) {
        return $this->$name;
      }
    }
}

class Child extends ParentClass
{
    private $privateProperty = 'inChild';
}

$parent = new ParentClass;
var_dump($parent->privateProperty);

$child = new Child;
var_dump($child->privateProperty);