PHP Method Overriding: Change a Parent Method

A child class can override a method: write a method with the same name and replace the parent’s behaviour.

index.php

PHP

<?php
class Animal
{
    public function speak(): string
    {
        return "Some sound";
    }
}

class Cat extends Animal
{
    public function speak(): string
    {
        return "Meow";
    }
}

class Cow extends Animal {}

echo (new Cat())->speak() . "\n";
echo (new Cow())->speak();

Output

OUTPUT

1
2

Meow
Some sound

Cat replaced speak(). Cow kept the parent’s version.

Extend, do not only replace: parent::

Often you want to add to the parent’s behaviour. Call it with parent::.

index.php

PHP

<?php
class Notification
{
    public function message(): string
    {
        return "You have a new notification.";
    }
}

class UrgentNotification extends Notification
{
    public function message(): string
    {
        return "URGENT: " . parent::message();
    }
}

echo (new UrgentNotification())->message();

Output

OUTPUT

URGENT: You have a new notification.

Polymorphism in action

Code that works with the parent type calls the right method automatically.

index.php

PHP

<?php
class Animal
{
    public function speak(): string { return "..."; }
}

class Dog extends Animal
{
    public function speak(): string { return "Woof"; }
}

class Cat extends Animal
{
    public function speak(): string { return "Meow"; }
}

foreach ([new Dog(), new Cat(), new Animal()] as $animal) {
    echo $animal->speak() . " ";
}

Output

OUTPUT

Woof Meow ...

This is explained further in Polymorphism.

Signature rules

An overriding method must be compatible with the parent’s:

It must accept at least the same parameters (it may add optional ones).
Its return type must be the same or narrower (more specific).
It may not make the method less visible (a public method cannot become protected).

index.php

PHP

<?php
class Base
{
    public function load(string $name): string
    {
        return $name;
    }
}

class Child extends Base
{
    public function load(int $name): string
    {
        return "changed";
    }
}

Output

OUTPUT

Fatal error: Declaration of Child::load(int $name): string must be compatible with Base::load(string $name): string in /path/to/index.php on line 12

The rule exists so that a Child can be used anywhere a Base is expected without surprises.

The Override attribute (PHP 8.3+)

Marking an override with #[\Override] makes PHP check that the parent really has that method. It catches typos and parent renames.

index.php

PHP

<?php
class Cat extends Animal
{
    #[\Override]
    public function speek(): string   // typo: a fatal error, because Animal has no speek()
    {
        return "Meow";
    }
}

On PHP before 8.3, the attribute is simply ignored (it is a comment to the engine), so it does no harm.

Preventing overrides

A method marked final cannot be overridden. See Final.

warning

Silent typos

Without #[\Override], a mistyped method name does not override anything. It creates a new method, and the parent’s one still runs. Be careful with names, or use the attribute.