PHP
Polymorphism
Learn to use different classes implementing the same interface as a common type.
Polymorphism
Polymorphism lets different classes that implement the same interface be used as a common type. When a Shape interface is implemented by both Circle and Square, each is a Shape.
A function can take the interface as a parameter: function describe(Shape $shape). Inside it calls $shape->area(); whether you pass a Circle or a Square, the same call works.
So code depends on the contract, not the concrete class. Adding a new Shape class doesn't change the describe function. This is the basis of open-extensible design.
An interface-typed parameter
When a function parameter is typed as an interface, you can pass any class implementing it. Example: function describe(Shape $shape): void { echo $shape->area(); }.
Now both describe(new Circle(2)) and describe(new Square(3)) are valid. The function doesn't know the concrete class; it only trusts that area() exists.
PHP checks whether the passed object implements the interface. If not, a fatal error is raised. This provides type safety.
Exercise A polymorphism example with Shape
First define interface Shape { public function area(): float; }. Then write class Circle implements Shape and class Square implements Shape. Each has a private float property (Circle: $radius, Square: $side), a __construct(float) and an area() method. Circle returns 3.14 * $this->radius * $this->radius, Square returns $this->side * $this->side. Then write function describe(Shape $shape): void { echo $shape->area() . "\n"; } and call describe(new Circle(2)) and describe(new Square(3)).
Show example solution
First define interface Shape. Write Circle and Square with implements Shape, each with a property + __construct + area(). Then write describe(Shape $shape) and call it with two objects.
Example solution:
<?php
// Shape interface and polymorphism example
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;
}
}
class Square implements Shape {
private float $side;
public function __construct(float $side) {
$this->side = $side;
}
public function area(): float {
return $this->side * $this->side;
}
}
function describe(Shape $shape): void {
echo $shape->area() . "\n";
}
describe(new Circle(2));
describe(new Square(3));
Test yourself
-
What does polymorphism enable?
- A class having multiple parents
- Using classes that implement the same interface as a common type
- Hiding properties
- Making methods private
Show answer
Using classes that implement the same interface as a common type — Correct. Polymorphism lets you use classes implementing the same interface as a common type.
-
Write an interface-typed parameter
function describe(____ $shape) {
echo $shape->area();
}
Show answer
function describe(Shape $shape) {
echo $shape->area();
}
-
Match the polymorphism expressions
- interface type
- Common parameter
- Circle, Square
- Implement the same contract
- describe($shape)
- Accepts both
- $shape->area()
- Single-form call
-
What is true for classes implementing the same interface?
- They cannot replace each other
- They can only be used as parent classes
- They can be used commonly via an interface-typed parameter
- Their method names must differ
Show answer
They can be used commonly via an interface-typed parameter — Correct. Classes implementing the same interface are used commonly via an interface-typed parameter.
Next lesson: interface vs abstract
