An abstract class is a class that cannot be instantiated directly — it exists solely to be extended. Abstract methods declare a method signature without providing an implementation, forcing subclasses to provide one.
abstract class ClassName.abstract public function name(): returnType; — no body.<?php
abstract class Shape
{
public function __construct(protected string $color = 'black') {}
// Subclasses MUST implement these
abstract public function area(): float;
abstract public function perimeter(): float;
// Concrete method — shared by all shapes
public function describe(): string
{
return sprintf(
'%s %s — Area: %.2f, Perimeter: %.2f',
ucfirst($this->color),
static::class, // late static binding
$this->area(),
$this->perimeter()
);
}
}
class Circle extends Shape
{
public function __construct(private float $radius, string $color = 'red')
{
parent::__construct($color);
}
public function area(): float
{
return M_PI * $this->radius ** 2;
}
public function perimeter(): float
{
return 2 * M_PI * $this->radius;
}
}
class Rectangle extends Shape
{
public function __construct(
private float $width,
private float $height,
string $color = 'blue'
) {
parent::__construct($color);
}
public function area(): float { return $this->width * $this->height; }
public function perimeter(): float { return 2 * ($this->width + $this->height); }
}
$shapes = [new Circle(5), new Rectangle(4, 6)];
foreach ($shapes as $shape) {
echo $shape->describe() . "
";
}
// new Shape(); // Fatal Error: Cannot instantiate abstract class
Tip: Use an abstract class when you want to share concrete code (methods, properties) alongside an enforced contract. If you need only the contract without shared implementation, use an interface instead.