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.

Practice

1. What is the correct way to access a constant defined in a PHP interface named MyInterface with a constant VERSION?

easy

A. MyInterface->VERSION

B. $this->VERSION

C. self::VERSION

D. MyInterface::VERSION

Solution

1. Step 1: Understand interface constants access

   Constants in interfaces are accessed using the interface name followed by double colon (::) and the constant name.

2. Step 2: Apply correct syntax

   Using MyInterface::VERSION correctly accesses the constant defined in the interface.

3. Final Answer:

   MyInterface::VERSION -> Option D

4. Quick Check:

   Interface constants use InterfaceName::CONSTANT [OK]

Hint: Use InterfaceName::CONSTANT to access interface constants [OK]

Common Mistakes:

• Using $this->CONSTANT inside interface context
• Using self::CONSTANT which refers to class, not interface
• Using arrow operator (->) with interface name

2. Which of the following is the correct way to declare a constant inside a PHP interface?

easy

A. const MAX_SIZE = 100;

B. define('MAX_SIZE', 100);

C. public const MAX_SIZE = 100;

D. var MAX_SIZE = 100;

Solution

1. Step 1: Recall interface constant declaration rules

   In PHP interfaces, constants are declared using const keyword without visibility modifiers.

2. Step 2: Identify correct syntax

   const MAX_SIZE = 100; is the correct way. Using define() or visibility keywords is invalid in interfaces.

3. Final Answer:

   const MAX_SIZE = 100; -> Option A

4. Quick Check:

   Interface constants use const without visibility [OK]

Hint: Use const without public/private in interfaces [OK]

Common Mistakes:

• Using define() inside interface
• Adding public/private before const
• Using var keyword for constants

3. Given the interface and class below, what will be the output?

interface Status {
    const ACTIVE = 1;
    const INACTIVE = 0;
}

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

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

medium

A. ACTIVE

B. 0

C. 1

D. Error: Cannot access constant

Solution

1. Step 1: Understand interface constants usage

   The interface Status defines constants ACTIVE = 1 and INACTIVE = 0. The class User implements this interface.

2. Step 2: Trace method getStatus()

   The method returns Status::ACTIVE, which is the integer 1. The echo statement prints this value.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Interface constant value printed = 1 [OK]

Hint: Interface constants accessed as InterfaceName::CONSTANT return their value [OK]

Common Mistakes:

• Expecting constant name string instead of value
• Trying to access constant via $this
• Assuming constants are not accessible in implementing class

4. What is wrong with the following PHP code?

interface Config {
    public const TIMEOUT = 30;
}

medium

A. Interface constants must be assigned string values only

B. Constants in interfaces cannot have visibility modifiers

C. Constants must be declared as static in interfaces

D. Constants cannot be declared inside interfaces

Solution

1. Step 1: Review interface constant declaration rules

   In PHP, interface constants must be declared without visibility keywords like public, private, or protected.

2. Step 2: Identify the error in code

   The code uses public const TIMEOUT = 30;, which is invalid syntax for interface constants.

3. Final Answer:

   Constants in interfaces cannot have visibility modifiers -> Option B

4. Quick Check:

   No visibility allowed on interface constants [OK]

Hint: Do not add public/private before const in interfaces [OK]

Common Mistakes:

• Adding public or private before const in interface
• Thinking constants must be static in interfaces
• Assuming constants only accept strings

5. You want to define an interface Shape with constants CIRCLE = 1, SQUARE = 2, and TRIANGLE = 3. Then, create a class Circle implementing Shape that returns the shape type constant. Which code snippet correctly implements this?

hard

A. interface Shape {
const CIRCLE = 1;
const SQUARE = 2;
const TRIANGLE = 3;
}

class Circle implements Shape {
public function getType() {
return Shape::CIRCLE;
}
}

B. interface Shape {
public const CIRCLE = 1;
public const SQUARE = 2;
public const TRIANGLE = 3;
}

class Circle implements Shape {
public function getType() {
return $this->CIRCLE;
}
}

C. interface Shape {
const CIRCLE = 1;
const SQUARE = 2;
const TRIANGLE = 3;
}

class Circle implements Shape {
public function getType() {
return self::CIRCLE;
}
}

D. interface Shape {
const CIRCLE = 1;
const SQUARE = 2;
const TRIANGLE = 3;
}

class Circle implements Shape {
public function getType() {
return Shape->CIRCLE;
}
}

Solution

1. Step 1: Define interface constants correctly

   Constants in interface Shape are declared using const without visibility modifiers.

2. Step 2: Access constants correctly in implementing class

   Inside Circle, access the constant using Shape::CIRCLE. Using $this->CIRCLE or self::CIRCLE is incorrect because constants belong to the interface, not the class instance or class itself.

3. Final Answer:

   Option A code snippet correctly implements interface constants and access -> Option A

4. Quick Check:

   Interface constants accessed as InterfaceName::CONSTANT [OK]

Hint: Access interface constants with InterfaceName::CONSTANT, not $this or self [OK]

Common Mistakes:

• Adding public before const in interface
• Using $this->CONSTANT to access interface constant
• Using self::CONSTANT inside class for interface constant
• Using arrow operator (->) with interface name