PHP Constants Explained: The Complete Guide

In PHP, constants are similar to variables, but with one critical difference: their values cannot be changed once defined. Constants are useful when you need immutable values throughout your script. In this Devyra guide, you’ll learn how to create and use constants effectively in modern PHP.

What is a PHP Constant?

A constant is an identifier for a fixed value. Unlike variables, constants:

• Do not begin with a $ sign
• Are global by default
• Cannot be modified or undefined once created

How to Create a Constant in PHP

Using define()

The most common method is with the define() function.

Syntax:

define(name, value);

Example:

define("GREETING", "Welcome to Devyra!");
echo GREETING; // Output: Welcome to Devyra!

Note: Constant names are case-sensitive by default.

Using the const Keyword

You can also define constants using the const keyword (outside of function or conditional blocks).

Example:

const MYCAR = "Volvo";
echo MYCAR; // Output: Volvo

const vs. define()

Constant Arrays (PHP 7+)

From PHP 7 onwards, you can define an array as a constant using define().

Example:

define("CARS", [
  "Alfa Romeo",
  "BMW",
  "Toyota"
]);

echo CARS[0]; // Output: Alfa Romeo

Array constants are especially useful for defining static configuration options.

Constants Are Automatically Global

Constants have a global scope. You can access them inside functions even if they were declared outside.

Example:

define("GREETING", "Welcome to Devyra!");

function displayGreeting() {
  echo GREETING;
}

displayGreeting(); // Output: Welcome to Devyra!

Conclusion

Constants in PHP offer a clean, reliable way to define fixed values that remain unchanged throughout script execution. Whether you’re using define() or const, they improve code readability, reduce bugs, and enforce good practices in configuration and data integrity.

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