What is the difference between abstract class and interface in PHP?

Question

What is the difference between an abstract class and an interface in PHP?

Answer

Abstract Class

• Can have both abstract (unimplemented) and concrete (implemented) methods
• Can have properties, constructor, and constants
• A class can extend only one abstract class
• Use when classes share common implementation

abstract class Shape
{
    public function __construct(protected string $color) {}

    abstract public function area(): float; // must be implemented

    public function describe(): string      // shared implementation
    {
        return "A {$this->color} shape with area " . $this->area();
    }
}

class Circle extends Shape
{
    public function __construct(string $color, private float $radius)
    {
        parent::__construct($color);
    }

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

Interface

• Only method signatures — no implementation (except default methods in some languages; not PHP)
• No properties (only constants)
• A class can implement multiple interfaces
• Use to define a contract that unrelated classes can all fulfil

interface Serializable
{
    public function serialize(): string;
    public function unserialize(string $data): void;
}

interface Loggable
{
    public function toLog(): array;
}

class Order implements Serializable, Loggable
{
    public function serialize(): string { return json_encode($this); }
    public function unserialize(string $data): void { /* ... */ }
    public function toLog(): array { return ['id' => $this->id]; }
}