OOP: Polymorphism (one call, many forms)

ep14_oop_polymorphism.php

<?php
interface Animals{
   public function speak();
}

class Dog implements Animals{
  public function speak(){
    return "The Dog Barks";
  }
}

class Cat implements Animals{
  public function speak(){
     return "The Cat Meows";
  }
}

class Bird implements Animals{
  public function speak(){
     return "The Bird Chirps";
  }
}

$animals = [new Dog(), new Cat(), new Bird()];

foreach ($animals as $animal) {
    echo $animal->speak();
}

Output

Output for this tutorial goes here.

The Dog Barks

The Cat Meows

The Bird Chirps

The Idea

Polymorphism means “many forms”. The same method call — $animal->speak() — behaves differently depending on the actual object behind it. One line, three different results.

How it works

Because Dog, Cat and Bird all share the Animals type, you can treat them the same way — store them in one array and loop over them — and each responds in its own way. PHP picks the right speak() at run time.

You’ve already used this: the checkout() function in the last episode ran the correct pay() for any payment class. That’s polymorphism.

Why it matters

You write code against the shared type, not each specific class — no long if/else chains checking “is it a Dog? is it a Cat?”. Add a new Animals tomorrow and the same loop handles it automatically.

The 4 Pillars of OOP

Encapsulation, Inheritance, Abstraction and Polymorphism — that’s the full set, and a classic interview question.

Example Output

Dog → The Dog Barks

Cat → The Cat Meows

Bird → The Bird Chirps

Key takeaway: polymorphism lets one method call take “many forms” — the same speak() does the right thing for each object, so your code stays simple and open to new types.

Interview notes: polymorphism shows up two ways — overriding (a child replaces a parent’s method) and interfaces / type-hints (many classes share one contract). Both let you swap objects without changing the calling code.