PHP Abstract Class: Base Classes with Abstract Methods

An abstract class is a class that cannot be created directly. It exists only to be extended. It can contain finished methods and abstract methods that child classes must write.

index.php

PHP

<?php
abstract class Shape
{
    abstract public function area(): float;

    public function describe(): string
    {
        return static::class . " with area " . round($this->area(), 2);
    }
}

class Circle extends Shape
{
    public function __construct(private float $radius) {}

    public function area(): float
    {
        return M_PI * $this->radius ** 2;
    }
}

class Rectangle extends Shape
{
    public function __construct(private float $w, private float $h) {}

    public function area(): float
    {
        return $this->w * $this->h;
    }
}

echo (new Circle(2))->describe() . "\n";
echo (new Rectangle(3, 4))->describe();

Output

OUTPUT

1
2

Circle with area 12.57
Rectangle with area 12

abstract class Shape cannot be instantiated.
abstract public function area() has no body. Every child must provide it.
describe() is a normal method that all children share. It calls area(), which each child defines in its own way.

You cannot instantiate it

index.php

PHP

<?php
abstract class Shape {}

try {
    new Shape();
} catch (Error $e) {
    echo $e->getMessage();
}

Output

OUTPUT

Cannot instantiate abstract class Shape

A child must implement all abstract methods

If a child forgets one, PHP reports a fatal error when the class is declared.

index.php

PHP

<?php
abstract class Shape
{
    abstract public function area(): float;
}

class Broken extends Shape {}

Output

OUTPUT

Fatal error: Class Broken contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Shape::area) in /path/to/index.php on line 7

The template method pattern

An abstract class can define the steps of a process and let children fill in the details.

index.php

PHP

<?php
abstract class Report
{
    public function render(): string
    {
        return $this->header() . "\n" . $this->body() . "\n" . $this->footer();
    }

    protected function header(): string
    {
        return "== Report ==";
    }

    abstract protected function body(): string;

    protected function footer(): string
    {
        return "== End ==";
    }
}

class SalesReport extends Report
{
    protected function body(): string
    {
        return "Sales: 1200";
    }
}

echo (new SalesReport())->render();

Output

OUTPUT

1
2
3

== Report ==
Sales: 1200
== End ==

Abstract class vs interface

	Abstract class	Interface
Can hold code	Yes	No (only method signatures)
Can hold properties	Yes	No
A class can use	Only one	Many
Use when	Related classes share code and a skeleton	Unrelated classes share a contract

The next lesson covers interfaces.

info

Why this matters in Laravel

Laravel’s base classes (Model, Controller, Command) are abstract or base classes. When you write class User extends Model, you inherit a large amount of behaviour and fill in only the parts that are yours.