PHP Constants Complete Guide | Define and Use Constants in PHP

Table of Contents:

PHP Constants Complete Guide | Define and Use Constants in PHP

At phponline.in, our goal is to make learning PHP easy, beginner-friendly, and SEO-rich. In this guide, we will cover everything about PHP constants — from what they are, how to define them, how they differ from variables, to advanced uses, magic constants, and best practices.

PHP Constants – Complete Beginner to Advanced Guide

You will Learn

Introduction to PHP Constants
Why Use Constants in PHP
Defining Constants Using define()
Defining Constants Using const
Rules for Naming Constants
Constants vs Variables in PHP
Case Sensitivity in Constants
Checking if a Constant is Defined
Using constant() Function
Magic Constants in PHP
List of All PHP Magic Constants
Global Scope of Constants
Constants in Classes (Class Constants)
Namespaces and Constants
Constants with Arrays
Best Practices for Using Constants
Performance Considerations for Constants
Common Mistakes with Constants
Real-World Examples of Constants

1. Introduction to PHP Constants

A constant in PHP is a name or an identifier for a simple value. Unlike variables, constants cannot be changed after they are defined.

Example:

2. Why Use Constants in PHP

Constants are used for values that should never change during script execution, such as:

Website name
API keys
Database credentials
Configuration values
Application-wide settings

3. Defining Constants Using `define()`

The define() function is the traditional way of creating constants in PHP.

Syntax:

Example:

4. Defining Constants Using `const`

The const keyword is another way to define constants, mostly used inside classes or for compile-time constants.

Example:

5. Rules for Naming Constants

Use uppercase letters with underscores: MAX_USERS
Cannot start with a number
No spaces allowed
Should be descriptive

6. Constants vs Variables in PHP

Feature	Constants	Variables
Can change value?	No	Yes
Start with `$`?	No	Yes
Scope	Global	Varies
Storage	Fixed	Dynamic

7. Case Sensitivity in Constants

By default, constants are case-sensitive. You can make them case-insensitive using define()‘s third parameter (deprecated in PHP 8.0).

8. Checking if a Constant is Defined

Use defined() function:

9. Using `constant()` Function

You can get a constant value dynamically:

10. Magic Constants in PHP

Magic constants change depending on where they are used.

11. List of All PHP Magic Constants

Constant	Description
`__LINE__`	Current line number
`__FILE__`	Full path and filename
`__DIR__`	Directory of the file
`__FUNCTION__`	Function name
`__CLASS__`	Class name
`__TRAIT__`	Trait name
`__METHOD__`	Method name
`__NAMESPACE__`	Current namespace

Example:

12. Global Scope of Constants

Constants are automatically global and can be accessed anywhere in the script.

13. Constants in Classes (Class Constants)

14. Namespaces and Constants

Constants can be defined inside namespaces:

15. Constants with Arrays

Since PHP 5.6, you can define array constants:

16. Best Practices for Using Constants

Use const inside classes
Group related constants together
Name constants descriptively
Avoid magic numbers

17. Performance Considerations for Constants

Constants are faster than global variables for fixed values
Avoid redefining constants in loops

18. Common Mistakes with Constants

Trying to change constant values after definition
Using undefined constants
Defining constants inside conditional blocks incorrectly

19. Real-World Examples of Constants

Example: Config File

PHP constants, PHP define, PHP const, PHP magic constants, PHP constant scope, PHP constant example, PHP define vs const, PHP constant naming

Learn more

Frequently Asked Questions (FAQs)

Q1: What is the difference between define() and const?
define() is used at runtime, const at compile-time and inside classes.

Q2: Are PHP constants case-sensitive?
Yes, unless explicitly made case-insensitive (deprecated in PHP 8.0).

Q3: Can a constant store an array?
Yes, from PHP 5.6 onwards.

Q4: Are constants faster than variables?
Yes, for fixed values, constants are slightly faster.

Q5: Can I unset a constant?
No, constants cannot be unset or redefined.