Constants in classes in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing class constant

What is the output of this PHP code?

PHP

<?php
class Fruit {
    const NAME = 'Apple';
}
echo Fruit::NAME;

A NAME

B Apple

C Error: Cannot access constant

D null

💡 Hint

Class constants are accessed with :: operator.

The constant NAME inside class Fruit is accessed using Fruit::NAME and outputs 'Apple'.

❓ Predict Output

intermediate

Output of constant in subclass

What will this code output?

PHP

<?php
class Vehicle {
    const WHEELS = 4;
}
class Bike extends Vehicle {
    const WHEELS = 2;
}
echo Bike::WHEELS;

A 2

B null

C Error: Cannot override constant

D 4

💡 Hint

Subclass constants can override parent constants.

Bike class overrides WHEELS constant with 2, so Bike::WHEELS outputs 2.

❓ Predict Output

advanced

Output of accessing constant with self and static

What is the output of this PHP code?

PHP

<?php
class ParentClass {
    const VALUE = 'Parent';
    public static function showSelf() {
        echo self::VALUE;
    }
    public static function showStatic() {
        echo static::VALUE;
    }
}
class ChildClass extends ParentClass {
    const VALUE = 'Child';
}
ChildClass::showSelf();
ChildClass::showStatic();

A ChildParent

B ChildChild

C ParentParent

D ParentChild

💡 Hint

self:: uses the class where method is defined; static:: uses called class.

self::VALUE refers to ParentClass::VALUE ('Parent'), static::VALUE refers to ChildClass::VALUE ('Child').

❓ Predict Output

advanced

Output of accessing class constant via object

What will this code output?

PHP

<?php
class Animal {
    const SOUND = 'Roar';
}
$animal = new Animal();
echo $animal::SOUND;

A Roar

B Error: Cannot access constant via object

C SOUND

D null

💡 Hint

Since PHP 5.3, you can access constants via object with :: operator.

Using $animal::SOUND accesses the constant SOUND of class Animal and outputs 'Roar'.

❓ Predict Output

expert

Output of constant expression with magic constant

What is the output of this PHP code?

PHP

<?php
class Test {
    const FILE = __FILE__;
    public static function showFile() {
        echo self::FILE;
    }
}
Test::showFile();

A Error: Cannot use magic constant in class constant

B __FILE__

C The full path and filename of the file where this code is saved

D null

💡 Hint

__FILE__ is a magic constant that gives the current file path.

Class constant FILE is assigned the magic constant __FILE__, which contains the full path of the current file.

Practice

1. What keyword is used to declare a constant inside a PHP class?

easy

A. let

B. var

C. const

D. static

Solution

1. Step 1: Understand constant declaration in PHP classes

   In PHP, constants inside classes are declared using the const keyword, which defines a fixed value that cannot change.

2. Step 2: Differentiate from other keywords

   var is used for properties (deprecated), let is not a PHP keyword, and static is for static properties or methods, not constants.

3. Final Answer:

   const -> Option C

4. Quick Check:

   Class constants use const keyword [OK]

Hint: Use 'const' to declare fixed values in classes [OK]

Common Mistakes:

• Using 'var' instead of 'const'
• Trying to use 'let' which is not PHP
• Confusing 'static' with constants

2. Which of the following is the correct syntax to access a class constant named VERSION in PHP?

easy

A. ClassName::VERSION

B. $obj->VERSION

C. ClassName->VERSION

D. $obj:VERSION

Solution

1. Step 1: Recall how to access class constants

   Class constants are accessed using the scope resolution operator :: with the class name, like ClassName::CONSTANT.

2. Step 2: Identify incorrect options

   $obj->VERSION tries to access a property, not a constant; ClassName->VERSION is invalid syntax; $obj:VERSION uses single colon which is invalid syntax.

3. Final Answer:

   ClassName::VERSION -> Option A

4. Quick Check:

   Access constants with ClassName::CONSTANT [OK]

Hint: Use ClassName::CONSTANT to access class constants [OK]

Common Mistakes:

• Using object operator -> instead of ::
• Trying to access constants as properties
• Confusing variable and class name usage

3. What will be the output of the following PHP code?

class Car {
    const WHEELS = 4;
}
echo Car::WHEELS;

medium

A. 4

B. WHEELS

C. Error: Undefined constant

D. null

Solution

1. Step 1: Identify the constant declaration

   The class Car declares a constant WHEELS with value 4.

2. Step 2: Understand the echo statement

   The code prints the value of Car::WHEELS, which is 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Class constant value printed = 4 [OK]

Hint: ClassName::CONSTANT prints the constant's value [OK]

Common Mistakes:

• Expecting the name 'WHEELS' to print
• Thinking it causes an error
• Confusing constants with variables

4. What is wrong with this PHP code?

class Book {
    const TITLE = 'PHP Basics';
}

$book = new Book();
echo $book::TITLE;

medium

A. Cannot access constant via object instance with ::

B. Constant TITLE is not declared correctly

C. Missing semicolon after constant declaration

D. Code will output 'PHP Basics' without error

Solution

1. Step 1: Verify constant declaration

   The class Book correctly declares const TITLE = 'PHP Basics';.

2. Step 2: Verify access syntax

   $book = new Book(); creates an instance, and $book::TITLE is valid syntax to access the class constant from an instance. The code outputs PHP Basics without error.

3. Final Answer:

   Code will output 'PHP Basics' without error -> Option D

4. Quick Check:

   $instance::CONSTANT is valid [OK]

Hint: ClassName::CONSTANT or $instance::CONSTANT both valid [OK]

Common Mistakes:

• Using -> for constants instead of ::
• Declaring with var instead of const
• Thinking $instance::CONSTANT is invalid

5. Given the class below, which code correctly prints the constant STATUS without creating an object?

class Server {
    const STATUS = 'Online';
}

hard

A. echo Server->STATUS;

B. echo Server::STATUS;

C. echo $server::STATUS;

D. echo Server::status;

Solution

1. Step 1: Identify correct constant access syntax

   Constants are accessed using the class name and the scope resolution operator :: with the exact constant name in uppercase.

2. Step 2: Evaluate each option

   Server->STATUS is invalid because -> is for object properties; $server::STATUS is invalid because $server is undefined and constants should be accessed via class name; Server::status is wrong due to case sensitivity; only Server::STATUS is correct.

3. Final Answer:

   echo Server::STATUS; -> Option B

4. Quick Check:

   Use ClassName::CONSTANT with exact case [OK]

Hint: Use ClassName::CONSTANT with exact uppercase name [OK]

Common Mistakes:

• Using object operator -> instead of ::
• Ignoring case sensitivity in constant names
• Trying to access constants via undefined variables