PHP Constant

In PHP, a constant is a value that cannot be changed during the execution of the script. Once defined, a constant's value remains the same throughout the script. Constants are usually defined using the define() function or, in PHP 7.0 and later, using the const keyword.

Defining Constants

Using define()

define("PI", 3.14159);
echo PI; // Output: 3.14159

In this example, PI is defined as a constant with a value of 3.14159. Once defined, PI cannot be changed.

Using const

const GRAVITY = 9.81;
echo GRAVITY; // Output: 9.81

Here, GRAVITY is defined as a constant with a value of 9.81. Similar to define(), once GRAVITY is set, it cannot be altered.

Key Points about Constants

1. Case Sensitivity: By default, constants are case-sensitive when defined using define(). However, with the const keyword, constants are case-sensitive by default and cannot be changed.

   define("GREETING", "Hello World");
   echo greeting; // Output: Notice: Undefined constant "greeting"
   

   In contrast:

   const GREETING = "Hello World";
   echo GREETING; // Output: Hello World
   

2. Global Scope: Constants are globally accessible across the script, meaning you can access them from any part of your code without needing to pass them as parameters or use global keyword.

   define("USERNAME", "admin");
   
   function printUsername() {
       echo USERNAME; // Output: admin
   }
   
   printUsername();
   

3. No $ Sign: Unlike variables, constants are defined without the dollar sign ($).

   define("MY_CONSTANT", "Some Value");
   echo MY_CONSTANT; // Output: Some Value
   

4. Use in Classes: When using const within a class, constants are accessed using the class name and the scope resolution operator (::).

   class MathConstants {
       const E = 2.71828;
   }
   
   echo MathConstants::E; // Output: 2.71828
   

Constants are useful for defining values that remain unchanged throughout the execution of your script, such as configuration values, mathematical constants, and other fixed data.