Class Constants – PHP OOP

Class Constants in PHP OOP – Complete Guide With Examples

Class constants play a crucial role in Object-Oriented Programming by allowing you to define values that should never change throughout the lifecycle of a PHP application. Constants are extremely useful when you want to store fixed values such as configuration labels, status codes, roles, error messages, and reusable identifiers in a secure and readable format.

This guide explains everything you need to know about Class Constants in PHP OOP, including syntax, declaration rules, use cases, and real-world examples.

What Are Class Constants in PHP?

A Class Constant is a value defined inside a class that cannot be changed once declared.
They are declared using the const keyword.

Unlike variables:

• Class constants do not use the $ sign
• Their values remain fixed
• They are accessed using the scope resolution operator (::)
• They can be accessed without creating an object

Example:

class MyClass {
    const VERSION = "1.0";
}

Why Use Class Constants in PHP OOP?

Class constants help you store fixed, unchangeable values that increase code readability and reduce errors. They are widely used in real-world applications for:

• API versioning
• User roles
• Error messages
• Application modes (LIVE, TEST)
• Database status flags
• System-wide constant values

Using class constants enhances performance, security, and consistency throughout large PHP projects.

How to Declare Class Constants in PHP (Best Example for Beginners)

To create a class constant:

class MathValues {
    const PI = 3.14;
}

Points to remember:

• Use const keyword
• Names are usually written in uppercase
• Values cannot change after declaration

How to Access Class Constants Using the Scope Resolution Operator

Class constants are accessed using the double colon (::) operator.

Example:

echo MathValues::PI;

Output:
3.14

You do not need to create an object:

$obj = new MathValues();
echo $obj::PI; // Valid but not recommended

Difference Between Class Constants and Class Properties

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Using Class Constants Inside Methods

You can access class constants inside class methods using self::.

Example:

class AppConfig {
    const APP_NAME = "PHPOnline";

    public function showAppName() {
        return self::APP_NAME;
    }
}

$config = new AppConfig();
echo $config->showAppName();

Class Constant Visibility in PHP (public, private, protected)

PHP 7.1+ allows constants to have visibility:

class Demo {
    public const A = 1;
    private const B = 2;
    protected const C = 3;
}

Rules:

• public → accessible everywhere
• private → only inside the same class
• protected → accessible in child classes

Real-World Example Using Class Constants in PHP OOP

class OrderStatus {
    const PENDING = "pending";
    const PROCESSING = "processing";
    const COMPLETED = "completed";
    const CANCELLED = "cancelled";
}

echo OrderStatus::COMPLETED;

Use cases:

• eCommerce order statuses
• Payment response codes
• User access roles
• API message types

Using Class Constants in Inheritance (Advanced PHP OOP Topic)

Class constants can be inherited just like methods and properties.

class ParentClass {
    const MESSAGE = "Welcome!";
}

class ChildClass extends ParentClass {}

echo ChildClass::MESSAGE;

Overriding Class Constants in Child Classes

PHP allows child classes to redefine constants:

class Base {
    const ROLE = "USER";
}

class Admin extends Base {
    const ROLE = "ADMIN";
}

echo Admin::ROLE;

Best Practices for Using Class Constants in PHP OOP

• Use uppercase names separated with underscores
• Group constants logically inside classes
• Use constants for fixed configuration values
• Avoid magic strings inside code
• Prefer constants over global variables
• Use visibility modifiers when needed
• Use class constants for enums (PHP 8.1+ offers native enums)

Learn more

• Learn more about PHP OOP Concepts on our homepage
• Read next: Class Properties in PHP OOP
• Explore: Static Methods and Properties in PHP

Frequently Asked Questions (FAQ)

1. What is a class constant in PHP?

A class constant is a fixed, unchangeable value defined using the const keyword inside a class.

2. How do I access a class constant?

Using the scope resolution operator:
ClassName::CONSTANT_NAME

3. Can class constants have visibility?

Yes. PHP supports public, private, and protected visibility for class constants.

4. Can I change a class constant after declaration?

No. Class constants are immutable.

5. Do I need to create an object to access a constant?

No. Class constants can be accessed directly without creating an object.

6. Can class constants be used inside methods?

Yes. Use self::CONSTANT_NAME inside class methods.

7. Can child classes override class constants?

Yes, but only by redeclaring them in the child class.