<?php

class Base {
  public function __toString() {
    return $this->name;
  }
}

class Concrete extends Base {
  private $name;

  public function __construct($name) {
    $this->name = $name;
  }
}

$o = new Concrete('foobar');
echo $o;

Fatal error: Cannot access private property Concrete::$name on line 5