<?php

class Pet {
  public $name;
  public $isRunning = false;

  public function run()
  {
    $this->isRunning = true;
    return $this->name . ' is running';
  }
}

class Dog extends Pet {
  protected $currentChaseTarget;

  public function chase(Pet $chaseTarget)
  {
    $this->isRunning = true;
    $this->currentChaseTarget = $chaseTarget;
    $this->currentChaseTarget->run();

    return $this->name . ' is chasing ' . $chaseTarget->name;
  }
}

$cat = new Pet();
$cat->name = 'Felix';

var_dump($cat);

$cat->run();

var_dump($cat);

$dog = new Dog();
$dog->name = 'Fido';

var_dump($dog);

var_dump($dog->chase($cat));

var_dump($dog);
var_dump($cat);