Constants cannot be changed after definition, while variables can; 1. Variables start with $, are mutable, scoped, and ideal for dynamic data; 2. Constants use define() or const, have no $, are immutable, globally scoped, and best for fixed values like configuration; 3. Use constants for unchanging values to improve readability, prevent errors, and centralize settings, but use variables when flexibility is needed; 4. const is for compile-time declarations in classes or global scope, while define() allows runtime or conditional definitions; 5. Avoid case-insensitive constants and using const in conditionals, as it causes parse errors, whereas define() is allowed; choosing the right one enhances code clarity, maintainability, and performance.

When working with PHP, understanding the difference between constants and variables is essential—especially when it comes to immutability. While both store data, their behavior, scope, and use cases differ significantly. Let’s break down the key differences and when to use each.

What Are Variables in PHP?

Variables in PHP are mutable containers for storing data. They can be changed at any point during script execution.

$name = "John";
echo $name; // Output: John

$name = "Jane";
echo $name; // Output: Jane

Key traits of variables:

• Start with a $ symbol (e.g., $name)
• Can be reassigned
• Scoped (local, global, static)
• Created on first assignment
• Case-sensitive

Variables are ideal when you need flexibility—like storing user input, loop counters, or temporary data.

What Are Constants in PHP?

Constants are immutable values that, once defined, cannot be changed or undefined during the script’s execution.

You define them using the define() function or the const keyword.

define("SITE_NAME", "MyWebsite");
echo SITE_NAME; // Output: MyWebsite

// Or using const (within a class or global space in PHP 7 )
const MAX_LOGIN_ATTEMPTS = 3;
echo MAX_LOGIN_ATTEMPTS; // Output: 3

Key traits of constants:

• No $ prefix
• Cannot be changed after definition
• Default scope is global (always accessible)
• Case-sensitive by default (though define() allows case-insensitive option)
• Must be assigned a value at declaration (can't be expressions in older PHP versions, but now allowed in const with compile-time values)

Key Differences: Constants vs. Variables

When to Use Constants

Constants are best for values that should not change and are used throughout your application. Examples:

• Configuration settings
• Application-wide settings
• Mathematical or algorithmic fixed values
• API keys (though better stored externally)

define("DB_HOST", "localhost");
define("PI", 3.14159);
const APP_ENV = "production";

Using constants here makes your code:

• More readable
• Less error-prone (no accidental reassignment)
• Easier to maintain (centralized values)

Important Notes and Best Practices

• Use const for class/interface/trait constants, and for compile-time constants in global space.

• Use define() for runtime definitions or when you need conditional definition.

  if (!defined('ENV')) {
      define('ENV', 'development');
  }

• Avoid case-insensitive constants (third argument in define()), as they can cause confusion:

  define('GREETING', 'Hello', true); // $greeting, $GREETING, $Greeting all work

• Constants cannot be used for storing resources or objects (in most cases).

• const must be used at the top level or inside a class; it cannot be used in conditional blocks:

  if (true) {
      const VALUE = 10; // Parse error!
  }

  But define() works here:

  if (true) {
      define('VALUE', 10); // OK
  }

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  Summary

  • Variables = flexible, changeable, scoped.
  • Constants = fixed, global, performant, and safer for unchanging values.

  Use constants when you want to enforce immutability and share configuration or fixed values across your app. Use variables when you need dynamic behavior.

  Choosing the right one improves code clarity, prevents bugs, and makes your PHP applications more maintainable.

  Basically: if it shouldn’t change, make it a constant.

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