Implementing with implements
Learn to implement an interface's methods in a class with implements.
The implements keyword
To declare that a class implements an interface, use the implements keyword: class Circle implements Shape { ... }.
The class must implement all methods declared in the interface with matching signatures. A missing method triggers a fatal error.
This binds the class to the interface's contract. From the outside, a Circle is a Shape and can be used wherever a Shape is expected.
A method implementation example
Let's write a Circle class that implements the Shape interface. First the interface: interface Shape { public function area(): float; }.
Then the class: class Circle implements Shape { public function area(): float { return 3.14 * $this->radius * $this->radius; } }. The class now defines area() with its own body.
From outside, $circle = new Circle() and calling $circle->area() runs Circle's own body. The contract is fulfilled.
Exercise Circle class implements the Shape interface
First define interface Shape { public function area(): float; }. Then write class Circle implements Shape with a private float $radius property and a __construct(float $radius) that runs $this->radius = $radius. The area() method does return 3.14 * $this->radius * $this->radius. Then create $circle = new Circle(5) and print echo $circle->area() . "\n".
Show example solution
float; }. Then inside class Circle implements Shape declare private float $radius, __construct and the area method.
Example solution:
<?php
// Define the Shape interface and implement Circle
interface Shape {
public function area(): float;
}
class Circle implements Shape {
private float $radius;
public function __construct(float $radius) {
$this->radius = $radius;
}
public function area(): float {
return 3.14 * $this->radius * $this->radius;
}
}
$circle = new Circle(5);
echo $circle->area() . "\n";Test yourself
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What is required when using implements?
- Implement all of the interface's methods with matching signatures
- Implement only one method of the interface
- Make the methods private
- Add properties to the interface
Show answer
Implement all of the interface's methods with matching signatures — Correct. implements requires all of the interface's methods with matching signatures.
-
Add the implements keyword
class Circle ____ Shape { public function area(): float { return 3.14; } }Show answer
class Circle implements Shape { public function area(): float { return 3.14; } } -
Match the implements expressions
- implements Shape
- Class accepts the contract
- area() method
- Written with a body in the class
- missing method
- Triggers a fatal error
- interface Shape
- The contract declaration
-
What happens if you don't write the interface's methods?
- A warning is issued and the code runs
- A fatal error is raised and the class is unusable
- Methods are auto-created with empty bodies
- Nothing happens, it is ignored
Show answer
A fatal error is raised and the class is unusable — Correct. implements requires all methods; a missing one raises a fatal error.