Constants in classes in PHP - Mini Project: Build & Apply

Constants in classes

📖 Scenario: Imagine you are creating a simple program to represent different types of vehicles. Each vehicle type has a fixed number of wheels that never changes. You want to store these fixed numbers as constants inside a class to keep your code organized and clear.

🎯 Goal: You will create a PHP class called Vehicle with constants for different vehicle types and their fixed number of wheels. Then, you will print out the number of wheels for a specific vehicle type using the class constant.

📋 What You'll Learn

Create a class named Vehicle.

Inside the class, define constants CAR_WHEELS with value 4 and MOTORCYCLE_WHEELS with value 2.

Outside the class, print the value of the constant CAR_WHEELS using the class name.

💡 Why This Matters

🌍 Real World

Constants in classes help keep fixed values organized and easy to maintain, like fixed settings or limits in a program.

💼 Career

Understanding class constants is important for writing clean, maintainable code in PHP, which is widely used in web development jobs.

1

Create the Vehicle class with constants

Create a class called Vehicle with two constants: CAR_WHEELS set to 4 and MOTORCYCLE_WHEELS set to 2.

PHP

<?php

// Create class Vehicle with constants CAR_WHEELS and MOTORCYCLE_WHEELS
// Your code here

Hint

Use the const keyword inside the class to define constants.

2

Add a variable to hold the vehicle type

Create a variable called vehicleType and set it to the string 'CAR_WHEELS'.

PHP

<?php

class Vehicle {
    const CAR_WHEELS = 4;
    const MOTORCYCLE_WHEELS = 2;
}

// Create variable vehicleType and set it to 'CAR_WHEELS'
// Your code here

Hint

Use a simple variable assignment with the exact string 'CAR_WHEELS'.

3

Access the constant using the variable

Create a variable called wheels and set it to the value of the constant in Vehicle class using the variable vehicleType. Use Vehicle::${vehicleType} syntax.

PHP

<?php

class Vehicle {
    const CAR_WHEELS = 4;
    const MOTORCYCLE_WHEELS = 2;
}

$vehicleType = 'CAR_WHEELS';

// Create variable wheels and set it to Vehicle::${vehicleType}
// Your code here

Hint

Use the constant() function to access a class constant dynamically.

4

Print the number of wheels

Print the value of the variable wheels using echo.

PHP

<?php

class Vehicle {
    const CAR_WHEELS = 4;
    const MOTORCYCLE_WHEELS = 2;
}

$vehicleType = 'CAR_WHEELS';
$wheels = constant('Vehicle::' . $vehicleType);

// Print the value of wheels
// Your code here

Hint

Use echo $wheels; to display the number of wheels.

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