Interface constants in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing interface constant

What is the output of this PHP code?

PHP

<?php
interface Vehicle {
    const WHEELS = 4;
}

class Car implements Vehicle {
    public function wheels() {
        return self::WHEELS;
    }
}

$car = new Car();
echo $car->wheels();

A 4

B Error: Undefined constant WHEELS

C 0

D Fatal error: Access to interface constant via self

💡 Hint

Remember that interface constants are public and can be accessed via the interface name.

Interface constants are public and can be accessed using self:: inside the implementing class because they are inherited as class constants. self::WHEELS looks for the constant in Car, which implements Vehicle, so it finds WHEELS and returns 4. Use Vehicle::WHEELS or self::WHEELS to access the constant.

❓ Predict Output

intermediate

Value of interface constant in class

What will be the output of this PHP code?

PHP

<?php
interface Status {
    const ACTIVE = 1;
    const INACTIVE = 0;
}

class User implements Status {
    public function getStatus() {
        return self::ACTIVE;
    }
}

$user = new User();
echo $user->getStatus();

A Error: Undefined constant ACTIVE

B 0

C 1

D Fatal error: Cannot access interface constant via self

💡 Hint

Check how interface constants are accessed inside implementing classes.

Interface constants are public and can be accessed using self:: inside the implementing class because they are inherited as class constants. self::ACTIVE looks for the constant in User, which implements Status, so it finds ACTIVE and returns 1. Use Status::ACTIVE or self::ACTIVE to access the constant.

❓ Predict Output

advanced

Output when accessing interface constant via instance

What is the output of this PHP code?

PHP

<?php
interface Shape {
    const SIDES = 3;
}

class Triangle implements Shape {
    public function sides() {
        return $this::SIDES;
    }
}

$triangle = new Triangle();
echo $triangle->sides();

A 0

B Error: Cannot access interface constant via instance

C Fatal error: Accessing constant SIDES via instance

D 3

💡 Hint

Interface constants must be accessed via the interface name.

$this::SIDES uses late static binding to access the constant in the Triangle class, which implements Shape and inherits the constant SIDES. Therefore, it returns 3. Interface constants are accessible via self::, static::, or $this:: inside the implementing class.

❓ Predict Output

advanced

Output of accessing interface constant with same name in class

What will this PHP code output?

PHP

<?php
interface Animal {
    const LEGS = 4;
}

class Dog implements Animal {
    const LEGS = 3;
    public function legs() {
        return self::LEGS;
    }
    public function animalLegs() {
        return Animal::LEGS;
    }
}

$dog = new Dog();
echo $dog->legs() . ',' . $dog->animalLegs();

A 4,4

B 3,4

C 3,3

D 4,3

💡 Hint

Class constants override interface constants with the same name when accessed via self.

self::LEGS inside Dog refers to Dog::LEGS which is 3. Animal::LEGS accesses the interface constant which is 4. So output is '3,4'.

🧠 Conceptual

expert

Why interface constants cannot be overridden in PHP

Which statement best explains why interface constants in PHP cannot be overridden by implementing classes?

A Interface constants are implicitly final and shared across all implementations to ensure consistent contract values.

B Implementing classes can override interface constants freely without restrictions.

C PHP does not allow constants in interfaces at all, so overriding is impossible.

D Interface constants are private by default, so they cannot be accessed or overridden.

💡 Hint

Think about the purpose of interfaces and their constants in contracts.

Interface constants define fixed values that all implementing classes must share to maintain a consistent contract. They are implicitly final and cannot be overridden to avoid breaking this contract.