Defining interfaces

Learn to define a contract with the interface keyword.

The interface keyword

An interface is a contract that lists the methods a class must implement. It looks like a class but holds no state (no properties); it lists only public method signatures and constants.

A method signature declares the method name and parameters but has no body { }; it ends with a semicolon. Every class that implements the interface must write these methods with its own body.

This way different classes follow the same contract and can replace each other. This is the foundation of polymorphism.

Interface syntax

An interface declaration starts with the interface keyword followed by a name: interface Shape { ... }. Method signatures go inside the braces.

Each signature looks like public function area(): float;; no body, ending with a semicolon. Even without public, methods are implicitly public.

An interface may also declare constants like const VERSION = '1.0';. But it cannot declare properties like $name; an interface holds no state.

Test yourself

What describes an interface?
- Method definitions together with method bodies
- Method signatures listed without bodies
- Holds properties and a constructor
- Is instantiated directly with new
Show answer

Method signatures listed without bodies — Correct. An interface lists only method signatures without bodies; it has no body or properties.

Write a method signature in an interface

interface Shape {
    ____
}

Show answer

interface Shape {
    public function area(): float;
}

Match the interface terms

interface

Contract
method signature

Bodyless declaration
public function

Default access
semicolon

Signature end
Which is NOT found in an interface?
- method signature
- class constant (const)
- property ($name)
- public method
Show answer

property ($name) — Correct. An interface cannot hold properties; it contains only signatures and constants.

Next lesson: Implementing with implements

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