Ulti's exercise

exercise.php

<?php
assert_options(ASSERT_ACTIVE, 1);
assert_options(ASSERT_WARNING, 0);
assert_options(ASSERT_QUIET_EVAL, 1);

function assert_handler($f, $l, $c) {
  echo "Assert failed at :$f:$l: $c";
  echo "\n<br />";
  exit();
}

assert_options(ASSERT_CALLBACK, "assert_handler");

// --------- Your code starts here ----------------

abstract class Animal {
  abstract public function sound();
}

class Dog extends Animal {
  public function sound() {
    return "woof";
  }
}

class Cat extends Animal {
  public function sound() {
    return "meow";
  }
}

// --------- Your code ends here ------------------

$dog = &new Dog;
$cat = &new Cat;

// Animal must have method sound()
assert(method_exists(Animal, "sound"));

// Dog must be a subclass of Animal
assert($dog instanceof Animal);

// Cat must be a subclass of Animal
assert($cat instanceof Animal);

// Dog cannot be a subclass of Cat
assert(!(Dog instanceof Cat));

// Cat cannot be a subclass of Dog
assert(!(Cat instanceof Dog));

// Dog must make the sound 'woof'
assert($dog->sound() == "woof");

// Cat must make the sound 'meow'
assert($cat->sound() == "meow");

echo 'Success!';

?>