Learn what PHP constants are, how to define them with define() and const, best practices, use cases and common pitfalls. Clear, practical guide for Indian developers.

Introduction

Constants in PHP are a simple but powerful tool for storing values that should not change during script execution. They help make code clearer, more maintainable and less error-prone — ideal for configuration values, API keys, fixed limits and labels. This guide explains what PHP constants are, how to define them using different approaches, practical examples, best practices and common pitfalls tailored for an Indian developer audience.

What are PHP constants?

A constant is an identifier for a simple value. Once defined, a constant's value cannot be changed or undefined for the duration of the script. Unlike variables, constants are globally accessible and have no dollar sign prefix.

  • Immutable: Constant values do not change after they are defined.
  • Global scope: Available anywhere in your script after declaration.
  • Types allowed: Scalars (string, int, float, bool) and arrays are commonly used as constant values.

How to define constants using define()

The most common way to create a constant is with the define() function. It's flexible and works at runtime.

<?php
// Basic usage
define('SITE_NAME', 'My Website');

echo SITE_NAME; // prints: My Website

// Define an array constant (supported in modern PHP runtimes)
define('SUPPORTED_LOCALES', ['en_IN', 'hi_IN']);

echo SUPPORTED_LOCALES[0]; // prints: en_IN
?>

Key points for define():

  • Names are usually uppercase by convention: SITE_NAME.
  • Constants defined with define() are available globally.
  • Use defined('NAME') to check existence before accessing.

How to define constants using const

The const keyword defines constants at compile time and is commonly used inside classes as well as in global scope.

<?php
// Global constant using const
const SITE_TAGLINE = 'Fast, Secure and Reliable';

echo SITE_TAGLINE;

// Class constant
class AppConfig {
    public const API_ENDPOINT = 'https://api.example.com';
}

echo AppConfig::API_ENDPOINT;
?>

Notes about const:

  • Cannot be used in conditional blocks at runtime (it's resolved earlier).
  • Works naturally for class constants and namespaced constants.
  • Visibility modifiers (public, private) may be available for class constants; check your runtime if needed.
What are PHP constants and how to define them?
Aspect Details
Definition An identifier for an immutable value available globally once defined.
Syntax Options Use define('NAME', value) or const NAME = value.
Typical Use Cases Configuration keys, API endpoints, fixed numeric limits, environment flags.
Data Types Scalars (string, int, float, bool) and arrays are supported for constants.
Scope Constants are globally accessible and do not require a dollar sign.
Class Constants Defined with const inside classes and accessed via ClassName::CONSTANT.
Checking Existence Use defined('NAME') or defined('ClassName::CONST') patterns to avoid warnings.
Best Practice Use descriptive uppercase names, group related constants into classes or namespaces for clarity.

Practical examples and tips

Here are some practical snippets and recommendations useful for web apps and APIs:

  • Store environment-specific values in constants loaded from a single config file: database names, default timezones, and upload limits.
  • Use class constants to group related constants, e.g., OrderStatus::PENDING, OrderStatus::COMPLETE.
  • Use defined() before defining a constant if your code can be included multiple times to avoid warnings:
<?php
if (!defined('MAX_UPLOAD_MB')) {
    define('MAX_UPLOAD_MB', 10);
}
?>

Common pitfalls

  • Relying on case-insensitive constant names is risky; use consistent uppercase naming.
  • Trying to change a constant value throws logic errors — use variables if mutability is needed.
  • Defining constants inside conditional blocks with const will cause errors; prefer define() when you need runtime conditional definitions.

Best practices for Indian developers

  • Keep configuration constants outside your webroot and load them via a secure include to avoid accidental exposure.
  • Use meaningful prefixes for shared or library constants to reduce name collisions (for example, MYAPP_ or module names).
  • Document constants in a central README or config file so teammates working across time zones can understand usage quickly.

Conclusion

Constants are a lightweight, dependable way to represent fixed values in PHP. Use define() for runtime flexibility and const when defining compile-time or class constants. Follow naming conventions, group related values, and check for existence when needed to write robust, maintainable code for web projects and APIs.

Frequently Asked Questions

Q: Can I change a constant after it is defined?

A: No. Constants are immutable for the duration of the script. If you need to change a value, use a variable or configuration storage.

Q: Which is better: define() or const?

A: Use const for compile-time definitions and class constants. Use define() for runtime or conditional definitions. Both have their place depending on context.

Q: Are constants case-sensitive?

A: Constant names are case-sensitive by default. Follow a clear naming convention like uppercase snake case to avoid confusion.

Q: Can I use arrays as constants?

A: Yes, modern PHP environments support array constants using both define() and const. Arrays can be useful for lists of supported locales or feature flags.

Q: How do I check if a constant exists?

A: Use defined('CONSTANT_NAME') before accessing the constant to avoid warnings and ensure safe loads when files may be included multiple times.