PHP Constants

PHP Constants – Complete Beginner Tutorial

While variables are designed to store values that may change during program execution, constants store values that remain the same. Once a constant has been created, its value cannot be modified or deleted during the execution of the script.

Constants are widely used in PHP applications to store configuration values such as website names, company information, API keys, version numbers, database settings, and file paths.

What is a Constant?

A constant is a named value that cannot be changed once it has been defined.

Think of a constant as information that should always remain the same throughout your application.

Examples include:

  • Your company name
  • Website URL
  • VAT or Tax Rate
  • Application Version
  • Database Host
  • API Keys

Creating a Constant using define()

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


<?php

define("SITE_NAME","PHP.Exalogics.com");

echo SITE_NAME;

?>

Output


PHP.Exalogics.com

Notice that constants are used without the $ symbol.

Constant Naming Rules

Rule Example
No dollar sign SITE_NAME
Usually uppercase MAX_USERS
Can contain numbers VERSION2
Cannot begin with a number 2VERSION ❌
Use underscores DATABASE_HOST

Why Use Constants?

Imagine your website contains your company name on hundreds of pages.

Instead of writing the name everywhere, define it once.


define("COMPANY","Travel & Culture Services");

Whenever you need it:


echo COMPANY;

If you ever need to change the company name, you only update it in one place.

Constants vs Variables

Variable Constant
Starts with $ No $ sign
Can change value Cannot change
Created using = Created using define() or const
Case-sensitive Usually uppercase

Using the const Keyword

PHP also allows constants to be created using the const keyword.


<?php

const VERSION = "1.0";

echo VERSION;

?>

Both define() and const are widely used in modern PHP.

Constants Inside Expressions

Constants can be combined with text just like variables.


<?php

define("SITE","PHP.Exalogics.com");

echo "Welcome to " . SITE;

?>

Output


Welcome to PHP.Exalogics.com

Checking Whether a Constant Exists

The defined() function checks whether a constant has already been created.


<?php

define("COUNTRY","Pakistan");

if(defined("COUNTRY"))
{
    echo "Constant exists.";
}

?>

Output


Constant exists.

Tip:

Using defined() helps prevent errors when including multiple configuration files.

Real World Example

Almost every professional PHP project has a configuration file that stores constants.


<?php

define("SITE_NAME","Travel & Culture");

define("SUPPORT_EMAIL","info@example.com");

define("PHONE","+92-321-1234567");

define("CURRENCY","USD");

?>

These values remain unchanged throughout the application and can be accessed from any included file.

Advantages of Constants

  • Prevent accidental modification.
  • Improve code readability.
  • Centralize configuration values.
  • Reduce repeated code.
  • Simplify application maintenance.
  • Useful for application settings and environment values.

Best Practice:

Use constants for values that should never change while your program is running, such as version numbers, application names, configuration values, and fixed URLs. Use variables for information that changes, such as user input, prices, or dates.

Magic Constants in PHP

PHP includes several built-in constants known as Magic Constants. Their values change depending on where they are used, making them extremely useful for debugging, logging, and determining file locations.

Magic Constant Description
__FILE__ Returns the full path and filename of the current file.
__DIR__ Returns the directory of the current file.
__LINE__ Returns the current line number.
__FUNCTION__ Returns the current function name.
__CLASS__ Returns the current class name.
__METHOD__ Returns the current class method.
__NAMESPACE__ Returns the current namespace.
__TRAIT__ Returns the current trait name.

Using __FILE__

This constant returns the complete path to the current PHP file.


<?php

echo __FILE__;

?>

Example Output:


/var/www/html/index.php

Using __DIR__

Returns the directory containing the current file.


<?php

echo __DIR__;

?>

This is especially useful when including other PHP files.


require_once __DIR__ . "/config.php";

Using __LINE__


<?php

echo __LINE__;

?>

PHP displays the line number where the statement appears.

This is useful when debugging large applications.

Constants Inside Classes

Classes can contain their own constants.


<?php

class Website
{
    const NAME = "PHP.Exalogics.com";
}

echo Website::NAME;

?>

Output:


PHP.Exalogics.com

Using Constants for Configuration

Most professional PHP applications have a configuration file.


<?php

define("DB_HOST","localhost");

define("DB_NAME","travel");

define("DB_USER","root");

define("DB_PASSWORD","password");

define("SITE_URL","https://php.exalogics.com");

?>

Every page can include this configuration file instead of repeating the same values.

Real World Example

Imagine you’re building an online booking system.


<?php

define("COMPANY","Travel & Culture Services");

define("EMAIL","info@travel-culture.com");

define("PHONE","+92-321-2424778");

define("DEFAULT_CURRENCY","USD");

echo COMPANY;

?>

If any of these values change in the future, you only update one file.

Common Beginner Mistakes

Mistake Correct Approach
Using $ before constants Constants never use a dollar sign.
Trying to change a constant Constants cannot be modified after creation.
Using lowercase names Use uppercase names for readability.
Using variables for configuration Use constants instead.

Best Practices

  • Use uppercase letters for constant names.
  • Group configuration constants into one file.
  • Use descriptive names.
  • Avoid hardcoding repeated values throughout your application.
  • Use __DIR__ for including files.
  • Keep sensitive configuration outside the public web directory whenever possible.

Practice Exercises

  1. Create a constant containing your website name.
  2. Create a constant for today’s currency.
  3. Create a constant containing your country.
  4. Display all constants using echo.
  5. Use defined() to check whether a constant exists.
  6. Display the current filename using __FILE__.
  7. Display the current directory using __DIR__.

Summary

Constants are ideal for storing values that never change while a PHP program is running. They improve readability, simplify maintenance, and reduce the risk of accidental changes. Magic constants provide additional information about your code and are invaluable for debugging and building professional applications.

Frequently Asked Questions

What is the difference between a variable and a constant?

A variable can change during program execution, while a constant keeps the same value for the entire script.

Do constants use a dollar sign?

No. Constants are referenced by name only.

Which is better, define() or const?

Both are valid. const is commonly used within classes and for simple declarations, while define() is often used in configuration files.

What are magic constants?

Magic constants are predefined constants that automatically return information such as the current file, directory, line number, class, or function.

Can constants store arrays?

Yes. Modern versions of PHP allow constants to contain arrays using the const keyword.

Quick Challenge

Create a file named config.php that contains constants for:

  • Website Name
  • Website URL
  • Support Email
  • Telephone Number
  • Current Version

Include this file in another PHP page and display all the constants.

Next Tutorial

Now that you understand variables, data types, and constants, it’s time to perform calculations and comparisons using PHP Operators.

Next: PHP Operators

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