PHP constant is a name or an identifier for a simple value that cannot change during the execution of the script. 

A valid constant name starts with a letter or underscore  $ sign can not use before constant define.

 Constants are case-sensitive by default.

PHP constants are defined using the define() function. Here's the basic syntax:

define(name, value, case-insensitive);

• name: The name of the constant.
• value: The value of the constant.
• case-insensitive (optional): Indicates whether the constant name should be case-insensitive. Default is false.

Here's an example of defining and using a constant:

<?php
define("PI", 3.14);

echo "The value of PI is " . PI;
?>

In this example-

The constant PI is defined with a value of 3.14. Once defined, you can use the constant in your code by referencing its name (in this case, PI).