<?php
class Mother
{
  protected function askForMoney()
  {
    print "May I please have some money?\n";
  }
}

class Daughter extends Mother
{
  protected function askForMoney()
  {
    print "Please, please, pretty please!\n";
  }
}

class Son extends Mother
{
  public function askToAskForMoney(Mother $personToAsk)
  {
    $personToAsk->askForMoney();
  }
}

$s = new Son;
$s->askToAskForMoney(new Mother);
$s->askToAskForMoney(new Daughter); //Valid Liskov substitution - should this fail?
?>