PHP Constants

Introduction

PHP Constant values should not be changed once defined. In PHP, such values are stored using constants.

Constants are very useful when you want to define values like configuration settings, database credentials, or fixed values such as PI or a website name. Unlike variables, constants cannot be modified once they are defined.

What is a PHP Constant?

A PHP Constant is a name or identifier for a simple value that cannot be changed during the execution of the script.

Example:

<?php
define("SITE_NAME", "TechFliez");
echo SITE_NAME;
?>

Output:

Note: Here, SITE_NAME is a constant, and its value cannot be changed later.

Why it is Used

1. Fixed Values

Used to store values that should not change (e.g., company name, API keys).

2. Security

Sensitive data like database credentials can be stored safely.

3. Code Consistency

Using constants avoids repeating the same value multiple times.

4. Global Access

Constants can be accessed anywhere in the script.

5. Readability

Makes code more understandable and organized.

Syntax

1. Using define() Function

The most common way to create a constant is using the define() function.

define("CONSTANT_NAME", "value");

Example:

<?php
define("PI", 3.14);
echo PI;
?>

Output:

2. Using the const Keyword

You can also define constants using the const keyword.

const SITE = "TechFliez";
echo SITE;

Output:

Difference between define() vs const:

• define() → Can be used anywhere
• const → Used inside classes or at the top level

Examples:

Example 1: Basic Constant

<?php
define("GREETING", "Hello Friends");
echo GREETING;
?>

Output:

Example 2: Multiple Constants

<?php
define("NAME", "John");
define("AGE", 25);
echo NAME;
echo AGE;
?>

Example 3: Using const

<?php
const COMPANY = "TechFliez";
echo COMPANY;
?>

Constant vs Variable

Real-Life Example

Example 1: Website Configuration

<?php
define("DB_HOST", "localhost");
define("DB_USER", "root");
define("DB_PASS", "1234");
define("SITE_NAME", "TechFliez");

echo "Welcome to " . SITE_NAME;
?>

In real-world applications:

• Database credentials are stored as constants
• Website name remains fixed
• Used in configuration files

Example 2: Mathematical Constant

<?php
define("PI", 3.14);
$radius = 5;
$area = PI * $radius * $radius;
echo $area;
?>

Note: Used in calculations where values should not change.

Common Mistakes

1. Using $ with Constants

❌ Wrong:

$PI = 3.14;

✔️ Correct:

define("PI", 3.14);

2. Trying to Change Constant Value

❌ Wrong:

define("PI", 3.14);
PI = 3.14159; // Error

3. Not Using Uppercase Names

✔️ Best practice:

define("SITE_NAME", "TechFliez");

4. Using Quotes While Accessing

❌ Wrong:

echo "SITE_NAME";

✔️ Correct:

echo SITE_NAME;

5. Defining Constant Twice

❌ Wrong:

define("PI", 3.14);
define("PI", 3.14159); // Warning

Conclusion

PHP constants are used to store fixed values that should not change during program execution. Unlike variables, constants provide better security, consistency, and readability.

Using constants for configuration settings, fixed values, and repeated data can significantly improve your coding practices.

PHP Constants – Interview Questions

PHP Constants – Objective Questions (MCQs)

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