PHP Constants

In PHP, constants are like variables, but their values cannot be changed once they are defined. Constants are useful for storing values that remain constant throughout the execution of a script, such as configuration settings, mathematical constants, or API keys.

PHP Constants

A constant is an identifier (name) for a simple value. The value cannot be changed during the script.
A valid constant name starts with a letter or underscore (no $ sign before the constant name).
Note: Unlike variables, constants are automatically global across the entire script.

Create a PHP Constant

To create a constant, use the define()function.

Syntax

define(name, value, case-insensitive);

• name: Specifies the name of the constant
• value: Specifies the value of the constant
• case-insensitive: Specifies whether the constant name should be case-insensitive. Default is false. Note: Defining case-insensitive constants was deprecated in PHP 7.3. PHP 8.0 accepts only false, the value true will produce a warning.

Example

Create a constant with a case-sensitive name:

                define("GREETING", "Welcome to W3Schools.com!");
                echo GREETING;

Example

Create a constant with a case-insensitive name:

                define("GREETING", "Welcome to W3Schools.com!", true);
                echo greeting;

PHP const Keyword

You can also create a constant by using the const keyword.

Example

                const MYCAR = "Volvo";
                echo MYCAR;

PHP Constant Arrays

From PHP7, you can create an Array constant using the define() function.

Example

Create an Array constant:

             define("cars", [
                "Alfa Romeo",
                "BMW",
                "Toyota"
              ]);
              echo cars[0];

Constants are Global

Constants are automatically global and can be used across the entire script.

Example

This example uses a constant inside a function, even if it is defined outside the function:

               define("GREETING", "Welcome to W3Schools.com!");

                function myTest() {
                  echo GREETING;
                }
                
                myTest();