startyourengines.php

<?php

class Car
{
  public function start()
  {
    /// wrooom
  }
}

class ElectricCar extends Car
{
  private $plugged = true;
  
  public function unplug()
  {
    $this->plugged = false;
  }
  
  // Will break clients 
  public function start()
  {
    if ($this->plugged) {
      throw new Exception('AAarrrg! Car is plugged in, please unplug first');
    }
    
    parent::start();
  }
}