What is Constant in PHP: Definition and Usage

In PHP, a constant is a name or an identifier for a simple value that cannot change during the execution of the script. Once defined using define() or const, its value remains fixed and cannot be altered.

How It Works

Think of a constant in PHP like a label on a jar that never changes. Once you put a label on a jar, you always know what's inside without opening it, and you can't change that label later. Similarly, a constant holds a value that stays the same throughout your program.

In PHP, you create a constant using define() or the const keyword. After setting it, you can use the constant anywhere in your code without worrying that its value will change by accident. This helps keep important values safe and clear.

Example

This example shows how to define and use a constant in PHP. The constant PI is set to 3.14 and then printed.

php

<?php
// Define a constant named PI
define('PI', 3.14);

// Use the constant
echo 'The value of PI is ' . PI;
?>

Output

The value of PI is 3.14

When to Use

Use constants when you have values that should never change, like configuration settings, fixed numbers (like mathematical constants), or status codes. This makes your code easier to read and safer because you avoid accidental changes.

For example, if you have a tax rate or a maximum number of login attempts, defining them as constants ensures these values stay consistent everywhere in your program.

Key Points

• Constants hold fixed values that cannot be changed once set.
• They are defined using define() or const.
• Constants do not use the $ sign when referenced.
• They improve code clarity and prevent accidental value changes.

Key Takeaways

Constants in PHP store values that never change during script execution.

Use define() or const to create constants.

Refer to constants without a $ sign.

Constants help keep important values safe and your code clear.

Ideal for fixed values like configuration, limits, or mathematical constants.